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► 



The Branner Geological Library 




ST/K?Giu3)-Wf/!ESSITY-U3MKif 



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DEPARTMENT OF THE INTERIOR 
UNITED STATES GEOLOGICAL SURVEY 

E OTI8 SMIxa, DittKcTott 



Pbofessional. Paper 75 



GEOLOGY AND ORE DEPOSITS 



BRECKENRIDGE DISTRICT, COLORADO 



FREDEEICK LESLIE EANSOME 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 
1911 



297945 



• * %• 



. • •• 

• a • • • 

 ft • 









• • • 

• • • 



• • • 



CONTENTS. 



Outline of report 9 

Chapter I. — Introduction 13 

Field work and acknowledgments 13 

Situation of the district '. •- 13 

Topography 15 

The general region 15 

Thediatrict • 16 

History of mining 16 

Production : 20 

Literature • 21 

Chapter II. — Pre-Cambrian and sedimentary rocks .•. 25 

Pre-Cambrian rocks 25 

Regional relations 25 

Local distribution 25 

Petrography 26 

Stratified rocks 26 

General stratigraphy of the region 26 

Hoosier Pass section .* 27 

Sedimentary formations of the Breckenridge district 30 

Formations represented 30 

"Wyoming*' formation -r.-f^rr ^^ 

Name 1 .I.j 31 

Distribution 31 

Thickness 31 

Lithology 31 

Cause of the red color 32 

Correlation 33 

Dakota sandstone 34 

Distribution 34 

Thickness 35 

Lithology 35 

Upper Cretaceous shale 38 

Preliminary statement 38 

Distribution 38 

Lithology 38 

Thickness 39 

Correlation 40 

Chapt^ III. — Petrography of the igneous rocks 43 

General features 43 

Earlier studies 43 

Relation to porphyries of near-by districts 43 

Types or varieties 44 

Quartz monzonite porphyry 44 

Megascopic character , • 44 

Microscopical character 44 

Chemical composition and classification 45 

Monzonite porphyry 50 

Megascopic character 50 

Microscopical character 51 

Chemical composition and classification. .; 53 

An exceptional facies 56 

Quartz monzonite porphyry (intermediate type) . ^ 57 

Distribution 57 

3 



72 



72 
72 



4 CONTENTS. 

Chapter III. — Petrography of the igneoas rocks — Continued. P^e. 
Quartz monzonite porphyry (intermediate type) — Continued. 

Megascopic features of the jwrphyry of Mount Guyot 57 

Microscopical features of the porphyry of Mount Guyot 58 

Chemical composition and classification of the porphyry of Mount Guyot 58 

Other facies • 59 

Relations of the porphyries to one another 60 

Chapter I V^. — Structure of the district 63 

Broad features 63 

Mosquito fault 64 

Eastern boundary of the Breckenridge tectonic block _ 64 

General structure of the district 65 

Dakotap overlap • 66 

Structural details '.... 67 

Relative ages of the porphyries 71 

Chapter V.— Quaternary deposits ^ 72 

Introductory statement 72 

Terrace gravels 

Distribution 1 . . 

Thickness 

Character of material 73 

Correlation with deposits outside of the district 74 

Older hillside wash 74 

Definition 74 

Distribution 75 

Thickness 75 

Character of material ^ 75 

Relative age of the terrace gravels and older hillside wash 76 

Moraines 76 

Distribution 76 

Moraines on Blue River 76 

Moraines in French Gulch 77 

Moraines on the Swan 7b 

Low-level gravels 78 

Occurrence 78 

Materials 79 

Glacial lake beds 79 

Recent alluvial and detrital deposits 79 

Chai*ter VI. — Mineralogy 81 

Introductory statement 81 

Native elements 81 

Sulphur 81 

Gold 81 

Crystallized gold of Farncdmb Hill 81 

Other occurrences of gold in veins 82 

Placer gold 83 

Silver K3 

Sulphides... 83 

Bismuthinite 83 

Galena 84 

Sphalerite 84 

Chalcopyrite 84 

Pyrite 85 

Oxides 85 

Quartz 85 

Hematite 86 

Magnetite 86 

Limonite 86 

Carbonates 87 

Calcite 87 

Siderite 87 

Smithsonite 88 



CONTENTS. 5 

Chapter VI. — Mineralogy — Continued. P«»e. 

Carbonates — Continued. 

Cerusi te 88 

Malachite 88 

Aznrite , 88 

Silicates 88 

Orthoclase 88 

Microcline 89 

Andesine and labradorite 89 

Hy perethene 89 

Augite and diopside 89 

Amphibole 89 

Garnet 89 

Zircon 89 

Epidote 90 

Allanite 90 

Muscovite ( sericite ) 90 

Biotite 90 

Chlorite 90 

Kaolinite 90 

Chrysocolla 90 

Titanosilicates 90 

Titanite 90 

Phosphates 90 

Apatite 90 

Sulphates 91 

Barite 91 

Gypsum 91 

Chapter VII. — ^Metamorphism 93 

Scope of chapter 93 

Metamorphism connected with the poiphyry intrusions 93 

Metasomatic changes in wall rock of ore deposits 94 

General considerations 94 

Alteration of monzonite porphyry of the Wellington mine 95 

Selection of material 95 

Comparison of chemical analyses 96 

Mineralogical changes 99 

Character of the solutions that effected the alteration 100 

Alteration of the quartz monzonite porphyry (silicic type) 100 

General character 100 

Chemical and mineralogical changes 101 

Propyli tization , 101 

Chapter VIII. — ^The primary ore deposits in general 103 

Mining 103 

General state of acti\aty and development 103 

Review of the northern and central parts of the area 103 

Vp French Gulch and over Famcomb Hill 103 

Down the Swan and back to Breckenridge by way of Gold Run and Gibson Hill 105 

Review of the southern mines 106 

Illinois Gulch 106 

Valley of the Blue south of Breckenridge 107 

Conclusion 107 

Tenor and treatment of the ores 108 

General character of the Assuring 110 

Classification and distribution of the deposits Ill 

Index to mining claims 112 

Chapter IX. — Veins of the zinc-lead-silver-gold series 124 

General features 124 

Plan of description 124 

Distribution 124 

Attitude 124 

Relation to faulting 124 

Width - 126 



6 CONTENTS. 

Chapter IX. — Veins of the zinc-lead-eilver-gold series — Continued. Paflre. 
Greneral features — Continued. 

Material 125 

Late movements 126 

Oxidalion 126 

Examples of zinc-lead-silver-gold deposits 126 

WeUinjcton mine 126 

. Introduction 126 

Underground development 127 

Geologic relations 127 

Attitudes and grouping of the veins 128 

Faulting 131 

Character and materials of the veins 133 

Oxidation of the ore 134 

Country Boy mine 134 

Introduction 134 

Underground workings 134 

Geologic relations 136 

Veins 136 

Helen mine 136 

Sallie Barber mine 137 

Little Sallie Barber mine 137 

French Creek tunnel 138 

Puzzle, Ouray, and Gold Dust mines 138 

Washington mine 140 

Jumbo mine 141 

Extension mine 142 

Little Corporal mine 142 

Chapter X. — Stockworks and veins of the gold-silver-lead series 143 

Distribution 143 

General structural features 143 

The ores 144 

FiXamples and detailed descriptions 1 U 

Jessie mine 144 

Introduction 144 

Undei^ground workings 144 

Geologic relations 146 

Ore bodies 146 

Hamilton mine - 147 

Cashier mine 148 

L X. L. mine 150 

Wire Patch mine 150 

Chapter XI.— The Famcomb Hill gold veins 153 

Distribution and geologic environment 153 

Underground workings 154 

Country rock 155 

The veins 155 

CBAVTKk XII. — Veins in the pre-Cambrian rocks and other deposits not described in the preceding chapters.. 168 

Veins in the pre-Cambrian rocks 158 

General features * 158 

Examples and mines 158 

Laurium mine 158 

Senator mine. * -• 159 

Arctic mine - 159 

Ling mine 160 

Blanket deposits of Gibson and Shock hills 160 

Gold-silver deposits in the Dakota quartzite 161 

Chapter XIII. —Some additional features of the deposits as a whole and their bearing on ore genesis 164 

Paragenesis of the ore-forming minerals 164 

Effects of various wall rocks on ore deposition 165 

Vertical variations in the ores* »..•.. 166 

Oxidation and enrichment 167 

Ground water 167 



CONTENTS. 7 

Chapter XIII. — Some additional features of the deposits as a whole and their bearing on ore genesis — Contd. Page. 
Oxidation and enrichment — Continued. 

Oxidation of lead-zinc ores 168 

Sulphide enrichment of lead-zinc ores 168 

Oxidation and enrichment of the Famcomb Hill veins 169 

Genesis 171 

Age of the deposits 174 

Chapter XIV. — Gold placrers 176 

Classification 175 

Gulch washings 175 

Bench placers 176 

Deep placers 177 

General features of the channels 177 

Material 178 

TenoT 179 

Drilling 180 

Dredging 180 

Chapter XV. — The geologic past and the economic future of the Breckenridge district 182 

Summary of geologic history 182 

Future of the district 183 

Index 185 



ILLUSTRATIONS. 



Plate I. Geologic map of the Breckenridge district, Colorado In pocket 

II. Map showing topography and mining claims in the Breckenridge district, Colorado In pocket. 

III. Outline map of the region adjacent to Breckenridge, Colo 16 

IV. A, Tenmile Range from road about a mile north of Boreas; B, Breckenridge and the Tenmile Range 

from Gibson Hill 18 

V. Ay Mount Guyot from Lincoln Park; B, French Gulch from Famcomb Hill 20 

VI. Quartz monzonite porphyry, silicic varieties, natural size: A^ Browns Gulch near Cashier mine; 

By East slope of Brewery Hill 44 

VII. Photomicrographs of porphyries: Ay Monzonite porphyry, near diorite porphyry, Wellington 
mine; By Quartz monzonite porphyry, east slope of Brewery Hill; C, Quartz monzonite por- 
phyry, slope east of Hoosier Pass 46 

VIII. Monzonite porphyry, natural size: Ay One mile northwest of summit of Mineral Hill; By Prospect 

Hill, 700 feet south of Abundance shaft 50 

IX. Photomicrographs of fresh and altered monzonite porphyry: A, Monzonite porphyry, near dio- 
rite porphyry, Wellington mine; By Same with nicols crossed; C, Same rock sericitized and car- 

bonatized near the vein 52 

X. Photomicrographs of porphyries: Ay Quartz monzonite porphyry (intermediate variety), east 
slope of Mount Guyot; By Quartz monzonite porphyry, slope east of Hoosier Pass; C, Monzo- 
nite porphyry, near diorite porphyry, Wellington mine 54 

XI. Quartz monzonite porphyry, intermediate varieties, natural size: A, Summit of Mount Guyot; By 

Ridge 2 miles southwest of Boreas 58 

XII. Geologic sections of the Breckenridge district 66 

XIII. Ay View north down the valley of the Blue from a point near the mouth of French Gulch; By Ter- 

race gravels on the east side of the Blue about a mile north of Breckenridge 68 

XIV. Ay View south up the valley of the Blue from the roadside three-quarters of a mile north of Breck- 

enridge j By Terrace gravels with Banner hydraulic workings on the west side of the Blue 70 

X V. ^, Terrace gravels of the Banner Placer, 1 mile north of Breckenridge; By Nearer view of the basal 

division showing the roundness and decomposition of the bowlders 72 

XVI. A, General view of the Gold Run placers from the southeast; By Near view of a bank in the Gold 

Run placers 74 

XVII. Ay Upper valley of the Blue with terminal moraine; By View from Famcomb Hill eastward up the 

middle Swan toward Swandyke and the Continental Divide 76 

XVIII. Ay Bank of the Gold Pan pit, showing character of deep gravels along the Blue; By Bowlders from 

the Gold Pan pit 78 



8 ILLUSTRATIONS. 

Page. 

Plate XIX. Native gold from Farncomb Hill 80 

XX. Native gold from Farncomb Hill 82 

XXL Native gold from Farncomb Hill 84 

XXII. Photomicrographs of metamorphosed sedimentary rocks: At Gamet^pidote rock with sulphides, 

south slope of Gibson Hill; B, Ore, Fox Lake tunnel, west base of Gibson Hill 92 

XXIII. Af Terrace gravels (Mekka placer) on south side of French Gulch as seen from Gibson Hill; 

B, Breckenridge and the Tenmile Range from Little Mountain 104 

XXIV. Ay Dredge on the Blue, near the mouth of the Swan; Bj View up French Gulch from Nigger 

Hill, showing mines and dredging operations...... 106 

XXV. Ore from Wellington mine, natural size: A, Incrustation of siderite in a vug in sphalerite; B, 

Veinlets of siderite in ore composed chiefly of dark sphalerite 124 

XXVI. Ore structures, natural size: -4, Spongy smithsonite in sphalerite, Sallie Barber mine; B, String- 
ers of sphalerite, pyrite, and galena in sericitized quartz monzonite porphyry , Jessie mine 126 

XXVII. Ay Wellington mine from the lower Country Boy tunnel; B, Farncomb Hill from the north. 128 

XXVIII. General plan of the principal underground workmgs of the Wellington mine 130 

XXIX. Ay The Jessie mine from the south; By Part of the Seminole open stope from the south 144 

XXX. General plan of the principal underground workings of the Wapiti group exclusive of those 

on the Fountain vein 164 

XXXI. Ay Naturally etched surface of Dakota quartzite, natural size; B, Ore from the Senator mine, 

near the head of Blue River 160 

XXXII. Sections of auriferous channels in the Breckenridge district, Colorado 178 

XXXIII. Ay Gold dredge working up the Swan from Valdoro; B, Another view showing the modern 

electric dredge working past the old stranded steam dredge 180 

FiouBB 1. Map showing approximate distribution of the principal silver, lead, and gold regions in Colorado. 14 

2. Characteristic texture of quartz monzonite porphyry (silicic type) as seen in thin section, with 

nicols crossed 44 

3. Characteristic texture of monzonite porphyry (calcic type) as seen in thin section, with nicols 

crossed 51 

4. Coarsely crystalline quartz monzonite porphyry (intermediate type) from the summit of Mount 

Guyot, as seen in thin section, with nicols crossed 58 

5. Diagram showing variation of molecular constituents in 18 intrusive rocks of the Breckenridge, 

Tenmile, and Leadville districts 61 

6. Diagram illustrating the general structure of the tectonic belt in which Breckenridge lies 63 

7. Generalized section across the Breckenridge region 64 

8. Diagrammatic sketch of a small sill of quartz monzonite porphyry intrusive into shale and quartz- 

ite on slope northeast of Lincoln 66 

9. Sketch showing replacement of shale by pyrite 85 

10. Diagram showing alteration of diorite- porphyry by vein-forming solutions 97 

11. Diagram illustrating gains and losses of each chemical constituent in terms of percentage of mass 

of original fresh diorite porphyry 98 

12. Geologic plan of the Oro and Extenuate levels of the Wellington mine 129 

13. Plan and section of the Country Boy mine 135 

14. Geologic section through the Helen tunnel 136 

15. Geologic section through the French Creek tunnel 138 

16. Geologic plan of the principal level of the Puzzle-Oura'y-Gold Dust workings 139 

17. Sketch section across a small ore body in the Gold Dust vein as stoped in 1909 140 

18. Plan of the principal levels of the Jessie mine 145 

19. Sketch of stringer lode in the May B. stope of the Jessie mine 146 

20. Diagrammatic section through the porphyry of the Jessie mine, showing the general character of 

the Assuring 147. 

21. Geologic sketch plan of the Tip Top tunnel of the Hamilton mine 148 

22. General plan of the Cashier mine 149 

23. Plan of the principal workings of the Wire Patch and Ontario mines, showing some of the geologic 

relations of the ore bodies 151 

24. Sketch plan of the lower Key West tunnel, showing character of Assuring 154 

25. Generalized section of a Farncomb Hill gold vein 156 

26. Plan of the principal level of the Laurium mine 159 

27. General plan of the underground workings of the Senator mine 160 

28. Plan of the Germania and South Elkhom tunnels in Little Mountain 162 

29. Sketch cross section showing mode of occurrence of a rich pocket of oxidized gold-silver ore in the 

quartzite of Little Mountain 163 



• • • « 

• • •• 






• •. 



GEOLOGY AND ORE DEPOSITS OF THE BRECKENRIDGE 

DISTRICT, COLORADO. '':/r 



By Frederick Leslie Ransome. 



OUTLINE OF REPORT. 

The Breckenridge district is situated in Summit County, Colo., 60 miles west-southwest 
of Denver, near the crest of the Continental Divide, and is drained by Blue River, a tributary 
of the Grand. Placer mining began here in 1860, but it was not until about 1879 that attention 
was turned to lode mining. During the last five years gold dredging, after many failures, 
has become established as an important industry. 

The production of the district can not be accurately determined. The placers may have 
yielded $10,000,000 or more and the lode mines probably much less. 

The fundamental rocks in the Breckenridge region are granites, pegmatites, gneisses, and 
schists of pre-Cambrian age. The thick series of Paleozoic rocks present in the Leadville and 
Tenmile districts thins to the north and west and is not represented near Breckenridge, where 
the oldest sedimentary rocks, resting directly on the pre-Cambrian, are the brilliantly red 
sandstones and shales of the ** Wyoming'' formation, supposed to be of Triassic or possibly of 
Permian age. Apparently conformable above them is the Dakota, locally a white quartzite 
with more or less gray shale, and this is overlain in turn by a thick formation of dark shales 
which probably represent the Benton, Niobrara, and part of the Montana formations of the 
Upper Cretaceous. In the northern part of the district the Dakota rests on the pre-Cambrian. 

The sediments and to some extent the underlying pre-Cambrian rocks are intruded by 
monzonitic porphyries ranging in composition from siliceous quartz monzonite porphyry to 
calcic monzonite porphyry or even to a hypersthene-bearing diorite porphyry. Three varie- 
ties are distinguished, of which two are mapped. The porphyries are probably derivatives of 
one magma and were erupted almost contemporaneously, although the quartz monzonite 
porphyry is younger than the more calcic variety. The intrusive bodies have invaded the 
sediments mainly as sills, but these are extremely irregular and in very many places the igneous 
rock cuts across the bedding of the sediments. The intrusions are most numerous and most 
irregular in the Upper Cretaceous shale. 

The bedded rocks incline generally eastward, the average dip being probably not far 
from 30**. On the west they lap upon the pre-Cambrian rocks of the Tenmile Range, on the 
other side of which are ''Wyoming" and older sediments of the Tenmile district faulted down 
against the old crystalline rocks by the Mosquito fault. On the east they are themselves 
dropped against the pre-Cambrian by the Mount Guyot zone of faults, so that one traveling 
from Breckenridge eastward toward the crest of the Continental Divide finds that the dark 
Upper Cretaceous shale instead of being overlain in that direction by younger beds is abruptly 
succeeded by the pre-Cambrian. The absence of the Paleozoic beds from the district and the 
disappearance of the ** Wyoming" formation northward, so that the Dakota lies directly upon 
the pre-Cambrian, are thought to indicate progressive transgression of the Paleozoic and 
Mesozoic seas on an ancient land mass which furnished much of the micaceous and arkosic 
material to the ** Weber," Maroon, and ** Wyoming" formations. In detail the structure is 





10 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

•. • • - 

complicated by the disturhffif^c?^ brought about by the intrusion of the porphyries and by 
faulting. ^ /•/ -\ '• ' 

The Quaterni^ry^{{^l)sits in the Breckenridge district may in part be divided into glacial 
accumulatioi^ of .l^leistocene age and stream gravels of the Recent epoch. During the Pleis- 
tocene tktsro \fare two advances and retreats of the ice and each has left a depositional record. 
'n\e eJE^rJifiP is represented by terrace gravels and by what has been called older hillside wash; 
/the iater by moraines and low-level gravels or valley trains. The terrace gravels are up to 
loo feet or more in thickness and extend from 250 to 650 feet above the present channel of 
' Blue River. They are composed of well-rounded and partly disintegrated material. The 
older hillside wash is up to 40 feet thick and contains angular fragments instead of waterwom 
bowlders. Both are auriferous and have been worked by the ordinary hydraulic method. 

A typical terminal moraine was formed across the valley of the Blue, just south of Breck- 
enridge, during the second glacial advance. Smaller moraines occur in French Gulch and 
along the three forks of the Swan. 

The low-level gravels are valley trains deposited by water from the melting ice during 
the second advance and retreat. They occupy the bottoms of the present valleys and are up 
to 90 feet thick. Ordinarily they range from 25 to 60 feet in thickness. Their material is 
generally well rounded and coarse, many of the bowlders along the Blue being over 6 feet in 
diameter near Breckenridge or up to 4 feet near Valdoro. The gravels in French Gulch and 
along the Swan are finer. The low-level gravels are being exploited by. dredging. 

Exclusive of the regional pre-Cambrian metamorphism the principal rock changes in the 
district are (1) local and erratic development of garnet, epidote, sulphides, and other minerals 
in the sedimentary rocks, probably as a direct result of the intrusion of the porphyries; (2) meta- 
somatic changes in the wall rocks of the ore deposits, especially in the porphyries; and (3) a 
prevalent propylitic alteration of the porphyries within a few hundred feet of the surface. The 
essential features of the wall-rock alteration in the monzonite porphyry are a notable decrease 
in silica, ferric oxide, lime, and alkalies accompanied by an increase in ferrous iron, magnesia, 
and water with the introduction of sulphides of iron, zinc, and lead and of a large quantity of 
carbon dioxide. Mineralogically these changes are expressed by the destruction of nearly all 
the original constituents of the rock and by the development of a ferruginous carbonate, 
sericite, and quartz, with some pyrite and sphalerite, in their place. This change was effected 
by waters rich in bicarbonates of iron, magnesium, manganese, and calcium, but poor in silica 
and alkalies. They carried also sulphur, zinc, and lead, with probably free carbonic and sul- 
phydric acids. In the siliceous quartz monzonite porphyry the change consists in the forma- 
tion of sericite with little or no development of carbonates. Water, sulphur, and heavy metals 
are added. The ore-depositing solutions at the places of deposition were evidently different 
in the two types of porph3rry. 

Propylitic alteration, involving the development of chlorite, epidote, quartz, and a little 
pyrite at the expense of the original minerals of the porphyries, appears to be restricted to 
rocks within 300 feet of the surface. It is thought to be the result of a modified or intensified 
kind of weathering" in which the chemical activity of the ordinary surface waters has been 
increased by the decomposition of sidphides. 

There were in 1909 only two mines, the Wellington and Country Boy, which were steadily 
producing ore in important quantities. The district in the past has yielded ores of varied 
kinds and richness, from the native gold of Famcomb Hill on the one hand to the zinc ore of 
the Country Boy on the other. A large proportion of the output has been partly oxidized 
argentiferous lead ore. The average value of the shipping ore and concentrates, at smelter 
prices, may be roundly estimated at about S25 a ton. 

The principal fissures in the district strike northeast. The dips are generally steep, the 
average being probably near to 65®. Those \^dth southeast dip predominate, but there are 
many with the opposite inclination. As a whole they form a conjugate system. No single 
fissure is known to exceed 1,700 feet in length and none was formed to the accompaniment of 
important structural displacement. 



/ 



OUTLINE OF REPORT. 11 

The primary deposits may be conveniently but to some extent artificially grouped as (1) 
veins of the zinc-lead-silver-gold series, (2) stockworks and veins of the gold-silveivlead series, 
(3) the gold veins of Famcomb Hill, (4) veins in the pre-Cambrian rocks, (5) metasomatic 
replacements along bedding planes, and (6) gold-silver deposits in quartzite. 

The Wellington veins constitute the chief examples of the zinc-lead-silver-gold series, whose 
members are closely associated Mrith the monzonite porphyry. These veins attain a maximum 
width of about 15 feet. The filling of the zinc-lead-silver-gold lodes consists chiefly of sul- 
phides, quartz in notable quantity being absent from most of them. In the Wellington mine 
the principal constituents are galena, sphalerite, and pyrite in various proportions. In the 
Country Boy the vein, where stoped in 1909, iS chiefly sphalerite. These minerals generally 
form a massive aggregate whose only banding is due to a later infiltration of siderite along 
small parallel cracks. Here and there the veins are displaced by north-south normal faults of 
slight to moderate throw. 

The ore deposits of the gold-silver-lead series are all in the northeastern half of the district 
and are as characteristically associated with the quartz monzonite porphyry as are the veins 
of the preceding group with the monzonite porphyry. A distinctive feature of these deposits 
is the occurrence of the ore in much-fissured and minutely veined rock, ordinarily quartz 
! monzonite porphyry, rather than in well-defined lodes. The pay shoots are masses of small 
stringers and impregnated rock and as a rule have no real walls. The type is illustrated by the 
Jessie mine. No deposit of this group was productive in the summer of 1909, although one 
(the Hamilton) was being reopened. The ores are of low grade and are generally pyritic, with 
varying quantities of sphalerite and galena. The veinlets making up the principal part of the 
ore bodies may contain a little quartz, but they are generally filled almost exclusively witli 
sulphides. These replace the porphyry to some extent, especially the large orthoclase 
phenocrysts. 

The gold veins of Farncomb Hill are remarkably narrow fissure veins that traverse Upper 
Cretaceous shale, with some sheets of porphyry, and contain isolated pockets of crystalline 
native gold in a limonitic matrix. The productive veins all lie on the north side of and near a 
considerable mass of quartz monzonite porphyry, which forms the core of the hill. The rich 
pockets are confined to a comparatively superficial zone, which probably nowhere exceeds 450 
feet in extreme depth from the highest point on the outcrop to the bottom limit of the ore. 
Their formation appears to have been connected with oxidation and was favored by the struc- 
ture of the shale, porphyry, and veins, particularly by the slight dislocation of the veins by 
slips along the bedding planes of the shale. In their original condition the veins probably con- 
tained pyrite, chalcopyrite, sphalerite, and galena in a calcitic gangue. 

The Famcomb HiU veins owe their rich pockets to the cooperation of sulphide enrichment 
followed by solution and segregation of the gold in the zonq of oxidation. 

The Breckenridge ores were deposited through the agency of thermal waters and gases 
given off from a solidifying monzonitic magma and perhaps cooperating with water of atmos- 
pheric or of less definitely assignable origin. Whether the metals in the ores came from the 
magma or from the rocks invaded by it is not known. The principal gaseous constituents 
of the magmatic waters were hydrogen sulphide and carbon dioxide. 

The ores were probably deposited in early Tertiary time and were enriched throughout 
the later Tertiary. 

The deposits in the pre-Cambrian rocks are generally rather narrow fissure veins carrying 
auriferous pyrite, in some veins with free gold, in a quartzose gangue. Associated with these, 
although not all occur in any one deposit, are sphalerite, galena, chalcopyrite, bismuthinite, 
magnetite, and specularite. Some of these are possibly pre-Cambrian in age. 

In Gibson and Shock hills certain beds in the Dakota have been partly or wholly replaced 
by ore in the form of blanket deposits. Apparently there are at least two such horizons in 
Gibson Hill which have in the past yielded oxidized or partly oxidized lead-silver ores, but no 
work has been done on them for some years. 



12 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

At a few places in -the district, especially on Shock Hill and Little Mountain, oxidized gold- 
silver ores have been found in the form of superficial pockets in the fissured Dakota quartzite. 
For the veins of the zinc-lead-silver-gold series the general sequence of deposition was (1) 
pyrite and sphalerite with probably some galena, (2) galena^ (3) siderite (or other iron-bearing 
carbonate) with small quantities of pyrite and sphalerite, and (4) calcite or barite (rare and 
nowhere abundant). Though this was the general order, there probably was much overlapping 
and some unrecorded repetition. Of the paragenesis of the other deposits little can be learned. 
The character of the deposits depends to a notable extent on the kind of country rock. 

[ The physical character of the Upper Cretaceous shale appears to have especially favored the 
accumulation of the gold pockets of Famcomff Hill. On the other hand, ores of the zinc-lead- 

I silver-gold series have not been found to any important extent in shale. The lodes of the zinc- 

' lead-silver-gold series are closely associated with monzonite porphyry, whereas the stockworks of 

' the gold-silver-lead series are just as intimately connected with the quartz monzonite porphyry. 

I Most of the mines show a decrease in the proportion of galena with augmented depth, 

which means in general a depreciation of the ore. Lead ores of shipping grade probably nowhere 

• in the district extend to a depth of much more than 300 feet. The downward change in the 
character of the ores indicates enrichment. - 

The surface of the ground water is very irregular. Its depth generally ranges from zero 
on low ground to about 200 feet on the ridges unless these are drained by adits. General 
oxidation appears to be confined to a zone about 200 feet deep and some residual galena may 
persist at the surface. The oxidized or partly oxidized ores are generally richer in lead, silver, 
and gold than the purely sulphide ores. The zinc is carried away and is probably deposited 
in part below the water level. A large part of the galena in the district is believed to have been 
concentrated by downward-moving atmospheric water. 

There are three general classes of gold placers — (1) the bench or high-level placers; (2) 
the deep or low-level placers; and (3) the gulch washings. The gulch washings were the first 
worked and yielded much gold to the pioneers in the district. The most noted placers of this 
group were on the slope of Farncomb Hill. The gold came originally from the veins in the hill. 
The bench placers are in the terrace gravels and older hillside wash. They have been extensively 
worked in the past by hydraulic methods, but were not being exploited in 1909. They are 

■generally of low grade. 

The deep placers, in the low-level gravels, occupy the bottoms of the present valleys and 
are now being worked by dredging. The pay channels are generally from 180 to 400 feet wide 
and average up to about 50 cents a cubic yard. There are spots richer than this, but much of 
the material worked does not yield 20 cents a yard. 



CHAPTER I. 
INTRODUCTION. 

FIEU> WORK AND ACKNOWLEDGMENTS. 

Geologic field work in the Breckenridge district began with a reconnaissance in the autumn 
of 1908, followed by detailed work frofai June 20 to September 24, 1909, by F. L. Ransome 
and E. S. Bastin. The topographic map Used by the geologists as a base had been made in 1908 
by Charles E. Cook and D. F. C. Moor. 

It would be impossible to acknowledge individually all the courtesies extended in further- 
ance of the work at Breckenridge, but appreciative mention may be made of the specially 
important services rendered in supplying maps and information, or in devoting time to under- 
ground guidance, by Messrs. T. A. Brown, F. C. Cramer, P. L. Cunmiings, R. W. Foote, O. K. 
Gaynor, M. M; Howe, Charles Kerns, H. W. Lohman, William Mitchell, John Nelson, Ben 
Stanley Revett, George C. Smith, and Charles G. Walker. Acknowledgment is due also to 
Dr. W. S. Ward, in charge of the mineral collection of the Colorado Museum of Natural History, 
in Denver, for his energy and interest in procuring for use in this report excellent photographs 
of some of the fine specimens of Farncomb Hill gold generously presented to the museum by 
Mr. John F. Campion, and to Mr. John W. Finch for courteously placing his unpublished report 
on the Wellington mine at my disposal. 

SITUATION OF THE DISTRICT. 

The portion of the Breckenridge district specially studied in connection with the present 
report and cartographically represented in Plates I and II (in pocket) covers about 45 square 
miles and includes parts of many irregular areas w^hose boundaries were early fixed by local 
regulations and which are individual mining districts in the narrower sense in which that term 
is used by land surveyors. The more important of these are the Bevan district, including the 
Wellington, Dunkin, Sallie Barber, and Little Sallie Barber mines ; the Union district, including 
the Country Boy, Jessie, Gold Dust, and Puzzle mines ; the California district, including Farn- 
comb Hill; and the Minnesota district, including the Laurium mine. The general position of 
the area embraced by Plates I and II is indicated by the small-scale outline maps of Plate III 
and of figure 1. In a broad sense the Breckenridge district includes more than the small rectan- 
gular* area here considered, the name being commonly applied to all of the country drained by 
the Blue and its tributaries above its confluence with Snake River and Tenmile Creek, near 
Dillon. It will frequently be convenient, however, to refer in general to this quadrangular tract 
as *' the Breckenridge district'* without denying the right of outlying territory to the same name. 

All of the district lies in Summit County, a little northwest of the central point of Colorado. 
The town of Breckenridge is 60 miles west-southwest of Denver in a straight line and 20 miles 
northeast of Leadville. By rail, owing to the rugged nature of the country, the distances from 
these cities are much greater, the length of the narrow-gage line of the Colorado & Southern 
Railway from Denver to Breckenridge being 110 miles, and from Breckenridge to Leadville 41 
miles. The position of Breckenridge in relation to the other metal-mining districts of Colorado 
•(exclusive of iron deposits) is shown in figure 1. 

13 



14 



GEOLOGY AND ORE DEPOSITS OF. BEECKENBIDGE DISTBICT, COLOBADO. 




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INTKODUCTION. 15 

TOPOGRAPHY. 
THE GENEBAL BEGION. 

The Breckenridge district (see PL III) lies about the headwaters of Blue River, on the west 
side of the main Continental Divide of the Rocky Mountain chain. The town of Breckenridge, 
at an altitude of 9,577 feet, is at an average distance of about 6 miles northwest of the divide, 
which from Argentine Pass, between Silver Plume and Montezuma, has a sinuous southwest 
course over Whale Peak (13,104 feet*), Georgia Pass (11,811 feet*),Mount Guyot (13,565 feet*), 
Bald Mountain (13,800 feet*), (Mount Hamilton on the Hayden map), Breckenridge Pass 
(Boreas) (11,503 feet'), to Hoosier Pass (11,627 feet'), 8 miles south of Breckenridge. At 
Hoosier Pass the divide turns west and passes north of Leadville across the linear uplift variously 
known as the Park, Mosquito, or Tenmile Range, and along Tennessee Pass to the Sawatch 
Range. 

A notable feature of the Continental Divide east of Breckenridge is the fact that much 
of it coincides with a gently rolling upland, which is evidently a remnant of a much older topog- 
raphy than that represented by the cirques and ravines notching its edges. S. H. Ball* describes 
what appears to have been originally a part of the same upland, in the Georgetown quadrangle, 
whose southwest comer (latitude 39^ 30', longitude 105® 45') lies 10 miles due east of the mouth 
of Georgia Gulch (PL I, in pocket). Ball concluded that this upland dates from late Tertiary 
time, which is probably a moderate estimate of its antiquity. 

On the west and north sides of the divide in the Breckenridge region heads Blue River with 
its tributaries. Snake and Swan rivers and Tenmile and French creeks. For 20 miles north from 
Hoosier Pass, to the point where its waters are joined by those of Snake River and Tenmile 
Creek, the Blue flows through a narrow but fairly open and partly gravel-filled, flat-floored 
valley, on whose west side the ground rises steeply, within a distance of 4 miles, to .the bold, 
serrate crest of the Tenmile Range (Pis. IV; V, B) — as the inhabitants term that portion of the 
Park Range lying mainly between Blue River and Tenmile Creek and culminating at 14,297 
feet in Mount Lincoln, southwest of Hoosier Pass. On the east a belt of mountainous country, 
widening northward and dissected transversely by French Creek and by Swan and Snake 
rivers, gives, as a whole, a more gradual ascent to the Continental Divide (PI. XVII, J8, p. 76), 
which is here the crest of the Colorado or Front Range. At Hoosier Pass, as already noted, the 
main divide crosses the line of the Blue, which heads in two small tarns just northwest of the 
pass. From the mouth of the Snake, on the other hand, the distance eastward to the Conti- 
nental Divide at the head of that stream, near Argentine Pass, is about 16 miles. 

From the mouth of Snake River the Blue flows nearly northwest for 35 miles and joins 
Grand River, one of the chief tributaries of the Colorado. The main Park Range continues to 
bound the valley of the Blue on its southwest side, and along this part of the range, northwest of 
Tenmile Creek, is locally distinguished as the Gore Mountains. 

On the south side of Hoosier Pass, nearly opposite the head of the Blue, is the source of the 
South Platte, which sweeps south and east across the broad, nearly circular upland basin known 
as South Park. Into this basin also are gathered, from Breckenridge and Georgia passes, 
other streams which unite to form Tarryall Creek, a tributary of the S6uth Platte. Still other 
tributaries from the vicinity of Whale Peak unite as the North Fork (of the South Platte), 
which after skirting the northern edge of South Park cuts through the Front Range in a deep 
canyon, followed by the railroad from Denver to Breckenridge, and joins the South Platte only 
about 8 miles above the point where that river emerges from the mountains to flow northward 
past Denver. 

Ten miles west of Hoosier Pass are the headwaters of the east fork of the Arkansas, which 
flows southward past Leadville. 

It appears from the foregoing description that the Breckenridge- district lies in the midst 
of a rugged part of the great Cordilleran chain, a gathering ground for rivers flowing east and 

1 EleTatioQfl detemUned by the Hayden Survey. 
> Elevations determln«d by the Wheeler Survey. 
* ProL Paper U. 8. OeoL Survey No. 63, 1906, pp. 31-32, Plate HI. 



16 GEOLOGY AND OBE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

west, into the Gulf of Mexico and into the Gulf of California. The region has the topography 
generally characteristic of the crest region of the Rocky Mountains throughout Colorado. The 
floors of the principal valleys range from 9,000 to 10,000 feet above sea level and are bounded 
by precipitous mountains of which a large number are between 13,000 and 14,000 feet in altitude, 
or rise from 3,000 to 4,000 feet above the valleys. The loftier peaks and ridges have been 
caived by local glaciation into pinnacles, ar6tes, and cirques, and the lower slopes and valleys 
have had many of their former diversities in relief subdued or concealed by accumulations of 
glfi<^ial detritus, partly in the form of moraines and partly as gravels reworked and deposited 
by streams. 

THE DI8TBICT. 

The part of the Breckenridge district mapped on Plate I ranges in altitude from about 
9,140 feet, on the Blue where that river leaves the area on the north, to 13,100 feet, on the 
northern spur of Bald Mountain, in the southeastern part of the district. The character of 
this relief, however, is not altogether representative of the general region, for it happens that the 
boundaries chosen for the present economic study include only a tract of relatively low ground 
adjacent to the confluences of the Swan and French Creek with the Blue. To the east, south, 
and west the mountains attain considerably greater heights than any of the elevations shown 
on Plate I. Thus the slope east of the upper part of French Creek rises precipitously within 
less than a mile to the summit of Mount Guyot (PL V, J.), which as already noted is 13,565 
feet high and is situated on the Continental Divide. Bald Mountain, just south of the spur 
shown on the map (PL I), has an elevation of 13,800 feet. The gentle slopes along the left 
bank of the Blue steepen rapidly within a mile to the west and sweep up to the crowning peaks 
and scarps of the noble Tenmile Range (Pis. IV; V, A), wliich easily dominates the landscape 
from any.point of outlook near Breckenridge. 

Though less rugged than much of the country about it, the area discussed m this report is 
in general decidedly hilly, having many steep slopes and little level land except along the larger 
streams. The topography is the expression of mature erosion, effected for the most part before 
the earliest locally recorded epoch of glaciation. The work of this erosive cycle is so far advanced 
that any peneplanation or other special features that might be ascribed to an earUer cycle 
have been obliterated and there is no recognizable remnant here of the old upland surface noted 
along the main divide to the east. The effects of glaciation are seen mainly in deposits of morainic 
material on the sides and bottoms of the present valleys and in the fact that Blue and Swan 
rivers and French Creek flow in aggraded channels, which in a few places are over 70 feet 
above their former rocky beds. Some topographic details especially relating to glaciation, 
such as the hummocky terminal moraine south of Breckenridge and the curious little alluvial 
flat known as Lincoln Park, will be treated in the chapter devoted to the work of the glacial 
epoch (pp. 76-79). Although, within the area here specially considered, the former presence of 
glaciers is recorded cliiefly by deposition, the characteristic erosive effects of the ice are well 
displayed in the Tenmile Range and along the crest of the Front Range. 

HISTORY OF MINING. 

Breckenridge, like Leadville and many other mining districts in Colorado, owes the dis- 
covery of its mineral* wealth to that wave of westw^ard emigration which in 1859 surged toward 
Pikes Peak and broke disastrously along the eastern flank of the Front Range. Not all, how- 
ever, who shared in the disappointment of that year turned their backs to the West; some 
penetrated the moimtains and found along the higher tributary streams sufficient gold to 
encourage them to remain. A number of these men began placer operations on Tarryall Creek 
and established the settlements of Hamilton and Tarryall, about 12 miles southwest of Brecken- 
ridge. Tarryall, now deserted, had a population of 2,000 to 3,000 in the early sLxties. Another 
party, consisting mainly of Georgians, ascended Michigan Creek, an affluent of Tarryall, pushed 



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INTRODUCTION. 17 

over the Continental Divide at }vhat is now known as Georgia Pass, and discovered the rich placer 
ground of Georgia Gulch, on the north side of Famcomb Hill. 

During the sixties placer mining was actively carried on in the gulches on the north slope 
of Famcomb Hill, and the town of Parkville, situated at the mouth of Georgia Gulch, is com- 
monly reported to have had at one time over 1,800 voters. A few timbers projecting from the 
tailings afterward washed over the site and some obscure graves in the pine-covered moraine 
fronting the mouth of the gulch are now the only relics of this town. According to W. P. 
Pollock,^ Georgia Gulch alone produced about $3,000,000 from its discovery to the close of 1862. 
Shortly after the settlement of Georgia and American gulches gold was found also on the French 
Creek side of the hill and for several years the Jeff Davis patch and other placers near Lincoln 
and the LilUan Vail placer in Nigger Gulch yielded bountiful returns. 

Other placers, including the rich and extensive ones of Gold Rim, which have probably 
yielded about $750,000, were opened as facilities for wt)rking them improved and Raymond 
records that in 1870 there were 100 miles of ditches and flumes in the coimty, most of these 
being in the Breckenridge region. At this time hand washing had for the most part given place 
to hydraulic methods, and booming, afterward extensively practiced, was then being introduced. 
The bed of French Gulch, especially in the neighborhood of Lincoln, was laboriously explored 
by drifting in these early days. 

In 1873 the country around Breckenridge was still almost as wild as when mining began. 
Thick timber covered the lower slopes of the mountains, roads were few, and the placers along 
the streams were the principal scenes of activity. The population of the whole county, which 
until 1883 included what are now Eagle and Garfield counties, did not exceed 350. Brecken- 
ridge at that time was reached by stage from Como or from Georgetown by way of the softly 
beautiful valley through whose verdant meadowland meanders Snake River in the curves 
that suggested its name. 

Silver-bearing lodes were opened on Glacier Mountain, near Montezuma, as early as 1864, 
but it was not until five years later that any attempt at lode mining was made near Brecken- 
ridge. In 1869 some argentiferous lead ore was taken from the Old ReUable vein, at Lincoln, 
and a small mill, run by an overshot waterwheel, was built on French Creek, close to the mouth of 
the tunnel. The Laurium mine (now sometimes called the Blue Flag), in Illinois Gulch, appears 
to have been opened about the same time'as the Old Reliable and supplied an argentiferous lead 
ore that was hauled in wagons to Denver and Golden. The Cincinnati (Robley claim), on 
Mineral Hill, near Lincoln, was developed in the early seventies and f^or over 10 years made 
shipments of high-grade galena and cerusite ore, said to have averaged about 65 per cent of 
lead and 16 ounces of silver to the ton. As early as 1873 a reverberatory furnace had been erected 
in French Gulch to treat this ore. Other lead-silver mines oj)erated on a small scale prior to 
1880 were the Union, Lucky, and Minnie. The Warrior's Mark ore body was discovered in the 
late seventies in the reddish sandstones northwest of Breckenridge Pass, where very little work 
sufficed to uncover an extraordinarily rich mass of silver ore carrying both lead and copper. 
This proved, however, to be a mere bunch or large pocket, close to the surface, and although 
much deep work, resulting apparently in the finding of some low-grade ore, was afterwards done 
in the vicinity of the original outcrop, these developments finally ended in disappointment. 

Notwithstanding the fact that rich placers had been washed on the slopes of Famcomb 
Hill since 1860, it was not until the end of 1879 or the beginning of 1880 that gold was found 
in place on the Ontario claim; this event was rapidly followed by discoveries on the Elephant, 
Boss, Key West, Bondholder, Gold Flake, and other now well-known claims on the hill. In 
view of the extreme narrowness of these veins and their failure to outcrop above their covering 
of soil the comparatively late date at which they were discovered is not altogether surprising. 
For about 10 years these wonderful little veins were actively exploited, chiefly by lessees, 
who riddled the northeast side of Famcomb Hill with tunnels and drifts (see PL XXVTI, 5, p. 1 28) 
and broke into pocket after pocket of the beautifully crystallized wire and flake gold for wliich 

1 Cited by Raymond, R. W., Statistics of mines and mining west of the Rocky Mountains, Washington, 1872, p. 328. 
90047*»— No. 75—11 2 



18 GEOLOGY AND OBE DEPOSITS OF BEECKBNBIDGE DISTRICT, COLORADO. 

this locality is justly famous. In 1885 there were over 100 men working on the hill; but by 
1890 the search had lost some of its zest; the masses of gold that had so often adequately 
rewarded years of labor were less frequently found, and the hill became gradually deserted by 
all except prospectors such as never recognize defeat or those who are willing to take the 
lessened chances of finding the residuum of gold that undoubtedly remains. Before the mining 
activity in this part of the district ebbed to its present low level, however, there was a period, 
between 1889 and 1898, when considerable work was done by the companies that succes- 
sively controlled what was originally known as the Ware property, after Col. A. J. Ware, one of 
the first to operate in the district on an extensive scale. Thus late in 1888 the Victoria Mining 
Co. built a mill in American Gulch and this was run for a few years on such low-grade ore as 
could be gathered from the workings and dumps on the north slope of Famcomb Hill. Another 
mill, now known as the Gold Dust mill, was built by the same company in 1889 on the west side of 
the Blue near Breckenridge and for a time nine or ten teams were kept busy hauling ore to it 
from the company*s mines, in which numerous lessees were at work. The total capacity of 
the two mills was 120 tons, but they were not long in operation together. About the year 
1894, the Victoria Mining Co. was succeeded by the Wapiti Mining Co., which built many miles 
of flume, bringing the water from the Middle Swan, and which hydraulicked many of the old 
dumps and much of the surface material on the north side of the hill, 

Anoth^er part of the district that for a time rivaled Famcomb Hill as a source of gold, 
although never noted for such beautiful specimens, is Gibson Hill. Here the first event of 
note was the discovery of the Jumbo lode by E. C. Moody in the summer of 1884, this being 
followed by active prospecting all over the hill. A settlement known as Preston was established 
on the north slope and for several years the Jumbo, Buffalo, Extension, and Little Corporal 
mines produced a large quantity of comparatively low-grade free-gold ore that was milled in 
part at Preston and in part at what was known as the Eureka mill, at the mouth of Cucumber 
Gulch, below Breckenridge. About the year 1886 Moody began work on the Seminole and other 
claims north of Gold Run, afterward developed into the Jessie mine. Before 1890 production 
had begun also from the Hamilton and Cashier mines. 

Meantime, while the gold deposits were being developed the silver-lead mines were not 
idle. In 1883 the Cincinnati was the largest mine on Mineral Hill, but it was soon surpassed 
by the Lucky. In the middle eighties Lincoln was a thriving town in which three small mills 
were active, treating about 60 tons of ore a day. In one of these was concentrated the first ore 
taken from the Oro mine, in 1887. In the following year the owners of the Oro built their own 
mill at the mine, which soon became one of the most productive in the district. Another mine 
that came into prominence about this time is the Iron Mask, situated on the west side of the 
Blue, in Shock Hill. From this mine shipments of high-grade silver-lead ore began in 1888 and 
continued with few interruptions for about 10 years. Other mines shipping in 1889 or 1890 
were the Ohio, on Shock Hill; the Kellogg and Sultana, on Gibson Hill; the Washington, Dunkin, 
and Juniata, on Nigger Hill; the Mountain Pride, at the head of Illinois Gulch; the Oro and 
Lucky, on Mineral Hill; the Victoria (Wapiti) group, on Famcomb Hill; and the I. X. L., 
on the Swan. Just beginning noteworthy development and production at this time were the 
Puzzle, Ouray, Country Boy, and Wellington mines. In 1891 the Boss and Gold Flake mines, 
on Famcomb Hill, yielded some rich masses of crystalline gold. Among the events of 1892 was 
the organization of the Jessie Gold Mining & Milling Co., which took over the property of the 
Gold Run Mining Co., and began the building of a new mill at the Jessie mine. The Extension 
Gold Mining & M il lin g Co. undertook in the same year the thorough development of what had 
hitherto been generally known as the ''Fair property,*' near the Jumbo mine. 

About the year 1896 shipments were resumed from the Mountain Pride, and this mine 
shortly afterward began extensive development and in 1898 was the leading producer in the 
district. This preeminence, however, was not long maintained and the mine had been idle for 
a number of years when visited in 1909. The Cashier and Jessie mines were actively worked 
and produced large quantities of milling gold ore in the late nineties, but work in both was 
abandoned some years ago. In 1909 the only mines producing were the Wellington, Country 



J. S. GEOLOGICAL 




TENMILE RANGE FROM ROAD ABOUT A MfLE NORTH OF BOREAS. 

the valle/ of the Blue and shows the peaks and cirques just north of the head of that si 
Quandary Peak appears at the left. See page i6. 




B. BRECKENRIDGE AND THE TENMILE RANGE FROM GIBSON I 

The distant gentle slopes on the left represent nearly the surface upon which the Paleozoii: sec 
Below them, near the town, is the terminal moraine on the Blue. The while dumps ne 
bowlders from the Gold Pan elevator pit. The wooded spur in the foreground, betw 
Breckenridge, is composed mainly of terrace gravel. See page ;>. 



INTRODUCTION. 19 

Boy, and Sallie Barber. The Hamilton mine, after being productive for many years, was . 
abandoned about the year 1902. It was reopened in 1909 and promises again to become impor- 
tant. During this same year also work was resimied in the Puzzle and Gold Dust workings, 
from which and from the Ouray mine large quantities of high-grade silver-lead ore with some 
gold were s toped during the 20 years following 1885. With these exceptions, all the old mines 
that have in the past contributed to the output from the district were uinproductive in 1909 and 
most of them were in an abandoned or ruinous condition. At some, such as the Washington 
mine, none of the workings could be entered. 

A visitor to the district, seeing the numerous deserted mines and forgetting that at no one 
time were all in simultaneous operation, is Ukely to exaggerate the difference between the 
present state of lode mining and former periods of more general activity. A review of the 
history of the district year by year, however, shows many vicissitudes. There have imdoubtedly 
been periods when the producing mines were far more mmierous than at present, when more 
mills were active, and when outlying settlements like Lincoln and Swan City had busy popula- 
tions. Yet not only did the hills, even in those prosperous times, display on their slopes the 
obvious traces of abandoned enterprises, but they also bore within them unsuspected riches 
whose discovery later was to give rise to fresh activity. 

The ordinary modes of placer mining, particularly hydraulic washing, booming, and bedrock 
drifting, continued to be actively practiced up to about the year 1900 and then gradually fell 
into disuse. In 1909 none of this work was in progress, except at one place on the upper Swan, 
where an attempt was being made to convert a bedrock drift into a sluice connected with an 
open pit; attention had been diverted from the high-level and superficial placers to those 
amenable to the modem method of dredging. 

It was in 1895 that Mr. Ben Stanley Revett, recognizing the possibility of working the deep 
gravels along the main streams, began by attempting to sink a shaft to bedrock on the Swan, 
near the mouth of Galena Gulch. This shaft, owing to the large quantity of water present, was 
not successful. He then uindertook to test the gravels with an oil drill, this probably being the 
first application of such an implement to prospecting in Colorado. 

In 1898 the American Gold Dredging Co., organized in Boston under the laws of Michigan, 
built two dredges on the Swan, but these, planned in accordance with New Zealand experience, 
proved imable to excavate the deep and coarse gravel near the mouth of Galena Gulch. In this 
year also the same company set up two Evans hydraulic elevators on the same stream. On the 
Blue, near the mouth of Cucumber Gulch, Pence & Miller were trying to sink a placer pit, using 
first hydraulic elevators and then steam pumps. At 30 feet in depth the pit had not reached 
bedrock. In 1899 the Blue River Gold Excavating Co. began work on the Blue, about 2 miles 
north of Breckenridge, with two dredges of the orange-peel type. These were failures. Toward 
the end of the year the North American Gold Dredging Co. built one Risdon and one Bucyrus 
dredge on the Swan and dismantled the two first constructed. The new boats, the larger having 
a capacity of 2,500 cubic yards a day, were operated for a few years, but were never fully suc- 
cessful and were finally abandoned. The gravels having been foimd diflicult to handle with the 
Ughtly constructed dredges then in use, the Gold Pan Mining Co., organized in December, 1899, 
with a capital of $1,750,000, acquired 1,700 acres of placer ground and undertook to work the bed 
of the Blue, at the south end of the town of Breckenridge, by using hydrauHc elevators. This 
company spent $750,000 in cash and gave 400,000 shares of stock, par value $1 each, to pay for 
the construction of 3 miles of 8-foot ditch and a connecting pipe line having a capacity of 6,000 
miner's inches, the erection of machine shops, and actual excavation. Bedrock was reached in 
October, 1902, at 73 feet. But this ambitious project, like its lesser predecessors, failed to wrest 
riches from the river channel, although it has left an enduring monument to itself as well as an 
instructive warning to others in the huge pile of bowlders, many of them over 6 feet in diameter, 
that now overlooks the town. (See Pis. IV, B, p. 18; XVIII, J8, p. 78.) In the prospectus 
issued by the company the productive life of the groimd had been estimated at 40 years, the 
average value of the gravel at 60 cents a yard, the cost of working at 10 cents a yard, and the 
total profits at $80,000,000. The plant has not proved entirely useless; part of the capacity of 



20 GEOLOGY AND QBE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

the ditch and pipe line has been utilized for lighting the town and the machine shops have 
proved a valuable adjunct to the gold-dredging industry. 

In 1905 the American Gold Dredging Co. was operating one dredge on the Swan, but this 
company appears to have done Uttle after that, and in 1907 its property was acquired by the 
Colorado Gold Dredging Co. In 1905 Mr. Revett, acting as trustee for the Reliance Gold 
Dredging Co., unincorporated, began the construction of a double-lift dredge on French Creek. 
This, although it was originally driven by steam, has been partly remodeled and is still in use. 
In 1908 the Colorado Gold Dredging Co. began dredging at Valdoro with two Bucyrus boats. 
One of these has been advancing steadily up the Swan, and the other, after going down this 
stream, has turned up the Blue. The latter was operated in 1909 but not in 1910. A fourth 
dredge, built by the French Gulch Dredging Co., was launched in French Gulch in the autumn of 
1908 and has since been successfully at work. It thus appears that in the establishment of gold 
dredging as an important and profitable industry the district has gained at least one very mate- 
rial offset to the decline, probably in part temporary, of some other forms of mining. 

This brief liistorical sketch would not be complete without a reference to the improved facili- 
ties for handling ore and concentrates afforded by the completion of the narrow-gage South 
Park & Pacific Railway, as it was then called, from Denver to Breckenridge in 1880, with its 
extension to Leadville in 1881, and to the advantages given to all kinds of mining by the intro- 
duction into the district during the last two years of electric power by the Central Colorado 
Power Co. and the Summit County Power Co. 

PRODUCTION. 

Complete statistics of production for the Breckenridge district or even for Summit County 
are not obtainable. For certain years. the output of Summit County is included with that of 
Park County in the statistical volumes of this Survey and of the Mint. Even where separate 
figures are published for Summit County it is impossible to distinguish the production of the 
Breckenridge district from that of the Robinson and Montezuma districts. Of late years most 
of the placer gold from Summit County has come from the Breckenridge district, which in 1908 
. yielded $146,098, or 79 percent of the total placer output of Colorado, amounting to $184,935. 
The published statistics, however, do not always give the placer yield of Summit County sepa- 
rately, this being combined in 1906, for example, with gold from siliceous ores, in order to con- 
ceal individual mine production. 

On February 28, 1885, the Summit County Journal published the following statement of 
placer production from Summit County: 

Placer gold produced in Summit County, Colo. 



Prior to 1870 $6, 500, 000 

1870 100, 000 

1871 70, 000 

1872 60, 000 

1873 101, 000 

1874 76, 500 

1875 '. 72, 500 

1876 150, 000 

1877 150, 000 



1878 $166, 000 

1879 100, 000 

1880 50, 000 

1881 60, 000 

1882 55, 000 

1883 105, 000 

1884 205, 000 



7, 021, 000 



It is not kno^vn how these figures were obtained nor how accurate they are, but inasmuch as 
Georgia Gulch alone is reported to have produced $3,000,000 by the close of 1862* and the 
Mint report for 1880 gives the placer production for that year as $50,000, the unknown statis- 
tician of the Summit County Journal does not appear to have exaggerated the output from the 
placers. The same local paper records in its issue of January 3, 1891, that 4,034 tons of ore 
and concentrates were shipped from Breckenridge in 1889 and 4,923 tons in 1890. The value 
is not given, but on the assumption that it averaged $25 a ton the output from mines would 

1 Raymoad, R. W., Statistics ot mines and mining, Wasblngton, 1872, p. 328. 



U. S. QEOLOaiCAL 8URVEV 




A, MOUNT GUYOT FROM LINCOLN PARK. 

The flat alluvial surface of the "park" is shown in the foreground. Beyond it is Fr 
which turns to the right of Mount Guyot, The lower slopes of the peak are shale 
down nearly to tirnber line, is porphyry. Sea page 57. 




n the distance is the Tenmile I 
deserted settlement of Lin. 
wooded slope behind it, « 
thinner than elsewhere, are 



FRENCH GULCH FROM FARNCOMB HILL. 

nge. The bold eminence to the right is Mineral Hi 



INTEODUCTION. 21 

be $100,850 in 1889 and $123,075 in 1890. In 1905 the total output of the district was about 
$280,000,^ and in 1907 the yield from seven mines (all that were productive) was $175,201.' In 
1908 the total production from the mines was $129,173,^ and in 1909 it was $538,704.' 

These fragmentary figures, unsatisfactory as they are, serve to indicate the relative produc- 
tivity of the district. At no time, unless perhaps during the first few years of shallow placer 
work, can this be called large. 

The output by metals in 1909 from mines and placers, as returned by the producers to 
this Survey, was as follows: 

Quantitus of metah produced in the Breckenridge district in 1909. 

Gold from placers. fine ounces. . 19, 574. 27 

Gold from ores do 1, 746. 48 

21, 320. 75 

Silver from placers fine ounces. . 5, 314. 00 

Silver from ores do 73,241.00 



• 



78, 555. 00 



Copper pounds. . 2, 508 

Lead do 3, 513, 698 

Zinc do. ... 5, 978, 167 

The gold and silver produced from the placers of Summit County during the last three 
years has virtually all come from the dredges working in the Breckenridge district. The 
figures, taken from the volumes of ^^ Mineral Resources" issued by this Survey, are as follows: 

Recent production from the Breckenridge placers. 



Year. 



1907. 

1908. 

i9ori, 



Gold (ounces}. 



1,800.23 

7,032.?7 

404,630.00 



Silver (ounces). 



662 
1,638 
5,314 



The notable increase in 1909 was due to the successful operation of new dredges. 

UTERATURE. 

The following are some of the important geologic publications relating to the Breckenridge 
region: 

Raymond, R. W., Statistics of mines and mining west of the Rocky Mountains, Washington, 1872, 1873, 1874. 
Notes on early history and development. 

Peale, a. C, Report on the geology of the South Park division: Seventh Ann. Rept. U. S. Geol. and Geog. Survey 
Terr., 1874, pp. 194-273. 

Dr. Peale ascended Mount Guyot from Georgia Pass. He describes the peak as formed of eruptive granite and gives 
a section (PI. VI, fig. 4) showing it separated from the Archean gneisses to the east by some altered shale and quartzite. 
The high hills south of Mount Guyot, between Michigan and Tarryall Creeks, are described as made up of a " porphyritic 
volcanic rock, approaching the character of a phonolitic trachyte." This is mainly the qiiartz monzonite porphyry of 
the present report. It is intrusive into Cretaceous shale which also forms a considerable part of the hills mentioned. 

On Tarryall Creek Dr. Peale noted quartzite (evidently the Dakota) dipping under the porphyry, and this is shown 
on the Hayden geologic map as extending continuously past Breckenridge Pass (Boreas) to the town of Breckenridge. 
Under the quartzite were noted red beds, supposed to be Triassic, forming the bedrock of the auriferous gravels of 
Tarryall Creek. 

Dr. Peale gives a detailed section of the beds exposed on the east slope of Silver Heels Peak, 10 miles south of Breck- 
enridge. All of Dr. Peale's work appears to have been south or east of the Breckenridge district. 

» Mineral Resources U. S. for 19a5, U. S. Geol. Survey, 1906. 

* Mineral Resourres U. S. for 1907, U. S. Geol. Survey, 1908, p. 140. 

> Returns from producers to the U. S. Geol. Survey. 



22 GEOLOGY AND ORE DEPOSITS OF BBECKENBIDGE DISTRICT, COLORADO. 

Marvinb, a. R., Report on the geology of the Middle Park division: Seventh Ann. Rept. U. S. Geol. and Geog. 
Survey Terr., 1874, pp. 184-192. 

Marvine ascended the valley of Blue River from Grand River, but devoted most of his attention to that part 
of the basin and its bounding ranges lying north of the confluence of Tenmile Creek and Snake River with the Blue, 
where is now the town of Dillon. He noted the occurrence of a thick series of apparently pre-Cretaceous sedimentary 
rocks in the vicinity of Hoosier and Breckenridge passes, but appears to have concluded that Archean rocks were exposed 
over a much laiger part of what is now called the Breckenridge district thaji is actually the case. The geologic map of 
northern-central Colorado (Sheet XII of the Hayden Atlas) shows extensive areas of Archean on both sides of the 
Blue near Breckenridge and on both sides of French Gulch from the vicinity of Lincoln to the pass at its head. The 
basin of the Swan also is represented as being entirely in the Archean (^'metamorphic granite "). 

Marvine concluded that the Cretaceous beds of the Williams Mountains, which lie east of the Blue between Snake 
and Grand rivers, are faulted down against the Archean rocks lying to the east of them, the throw being estimated at 
7,000 feet at least. Southward the fault, according to Marvine, '^appears to form the eastern side of the valley of the 
Blue for some distance, while it may be the northern continuation of some of the great faults that occur in the 
neighborhood of Mount Lincoln. *' The distribution of the Dakota sandstone, as shown on the Hayden map, however, 
natives the suggestion of any important fault extending from the Williams Mountains past Breckenridge to the 
vicinity of Mount Lincoln. 

Emmons, S. F., Geology and mining industry of Leadville: Mon. U. S. Geol. Survey, vol. 12, 1886, with atlas. 

The region particularly described by Mr. Emmons lies southwest of the Breckenridge district, but his more detailed 
descriptions are introduced (Chapter II) by a general account of the geology of the Mosquito (or Tenmile) Range and 
many of the rocks dealt with in his report occur also near Breckenridge. Some familiarity with the Leadville mono- 
graph is thus a prerequisite to any thorough study of the Breckenridge area; but it is too extensive a work to siunmarize 
here, and the interested reader should consult the original. 

Cross, Whitman, The laccolitic moimtain groups of Colorado, Utah, and Arizona: Fourteenth Ann. Rept. U. S. 
Geol. Survey, pt. 2, 1894, pp. 222-224, 227. 

Describes the intrusive porphyries of the Tenmile district, which Cross regards as practically all slight variants of 
one chemical type, the conspicuously porphyritic variety (Lincoln porphyry) being considered as differing only in 
texture from others that grade into homblendic facies, nearly free from quartz. Refers to the Dakota, '^probably with 
a thin layer of upper Jura below i t, " resting on the Archean in Middle Park, * * at the eastern base of the Mosquito Range, 
opposite the Tenmile district." The porphyries of the Tenmile district are older than the Mosquito fault, which, on 
the Hayden map, is shown faulting the Triassic and Jurassic but not the Dakota. Cross, however, points out that 
porphyries similar to those of the Tenmile district cut Cretaceous rocks at Blue River Butte and suggests thaf the 
Mosquito fault is probably yoimger than the Jurassic. 

Emmons, S. F., Tenmile district special folio (No. 48), Geol. Atlas U. S., U. S. Geol. Survey, 1898. 

Describes the geology and ore deposits of an area of 55 square miles adjacent to Kokomo, a town 10 miles west of 
Breckenridge. The sedimentary formations recognized are (1) the "Sawatch" quartzite, of Cambrian age, resting on 
the Archean gneiss, granite, and schist; (2) the Yule limestone, of Silurian age; (3) the Leadville limestone; (4) the 
"Weber formation;" and (5) the Maroon formation, all three of Carboniferous age; and (6) the "Wyoming" formation 
of ** Juratrias " age. 

The Archean rocks constitute a crystalline complex, the granites, gneisses, and schists being traversed by irregular 
intrusions of pegmatite and cut by eruptive rocks of post- Archean age. 

The "Sawatch " quartzite is white and evenly bedded with a fine-grained siliceous conglomerate at its base. The 
thickness of the formation ranges from 160 to 200 feet. Fossib belonging to a DikellocephaliLa fauna of Upper Cambrian 
age have been found in some argillaceous and calcareous shales in the upper part of the formation. 

The Yule limestone (not exposed within the area mapped) is characterized as a series of light drab-colored, gen- 
erally rather thin-bedded limestones, which are in many places magnesian and are everywhere more or less siliceous, in 
some places passing into calcareous sandstones. Their total thickness ranges from 120 to 160 feet. Conformably 
overlying the Yule limestone is the so-called Parting quartzite, from 15 to 60 feet thick, provisionally included with 
the Silurian beds. Near Leadville there is evidence of an imconformity between the "Parting" quartzite and the 
succeeding formations. 

The Leadville limestone (blue limestone) of Mississippian age, is the principal ore-bearing formation of Leadville, 
Red Cliff, and Aspen. In the Mosquito Range it is a dolomite, bluish gray or black near the top and lighter colored 
near the base. It has a granular texture and is generally thick bedded. It contains characteristic dark cherty con- 
cretions, is fossiliferous near the top, and passes upward into alternating shale and sandstone. The average thickness 
is about 200 feet, but the formation is not exposed in the Tenmile district. 

The "Weber formation " includes a lower calcareous shale member, called in the Leadville monograph the "Weber 
shales,'' which is transitional between the Leadville limestone and the main part of the "Weber formation," called the 
"Weber grits" in the Leadville report. This lower member is assumed to be about 300 feet thick in the Tenmile 
district and contains Pennsylvanian fossils. The upper member consists of coarse micaceous sandstones or grits, with 



INTEODUCTION. 23 

vubordinate beds of ahale and dolomitic limestone, the latter being important as containing a laige class of the ore 
deposits of the Tenmile district. The prevailing color is light gray and the thickness of the sandstone member is 
about 2,500 feet. 

The Maroon formation conformably overUee the ''Weber formation," which it in many respects resembles. It 
differs from the ** Weber" chiefly in the nonmagnesian character of its included limestone beds and in being generally 
of a reddish color. The limestones have afforded fossils of upper Pennsylvanian age. The total thickness of the 
Maroon formation is given as 1,500 feet. 

The "Wyoming'' formation lies conformably on the Maroon and consists chiefly of sandstones with some inter- 
calated shale, the whole being of a bright brick-red color. The sandstones are composed mainly of Archean debris 
and in many places are conglomeratic. Limestone is less abundant than in the two preceding formations. The total 
thickness of the ''Wyoming" is said to be 1,500 feet. 

The Archean rocks and overlying beds are cut by dioritic porphyries of which three varieties are distinguished on 
the geologic map, namely, the Lincoln, Elk Moimtain, and Quail porphyries. Under Elk Mountain porphyry are 
included various kinds transitional between the Lincoln and Quail types. The Lincoln type is characterized by large 
phenocrysts of orthoclase and smaller ones of quartz. It is suggested that the rock is in part a granite porphyry. The 
Elk Mountain porphyry at the type locality is similar to the Lincoln porphyry but does not contain the large orthoclase 
phenocrysts. The Quail porphyry is decidedly homblendic without noticeable megascopic quartz. The prevailing 
color is dark green. All the porphyries would probably have been classed with quartz monzonite and monzonite had 
the folio been written a few years later. The porphyries occur principally as intrusive sheets in the sedimentary 
formations and the number and regularity of these igneous intercalations are striking features in the structure of the 
district. There are also some bodies of rhyolite in the district that are much younger than the dioritic porphyries. 

The sediments and porphyry sheets are gently flexed and are faulted down along the west base of the Mosquito or 
Tenmile Range by the Mosquito fault which may have been initiated in pre-Oretaceous time. 

The ore deposits of the Tenmile district occupy fissures in the sedimentary beds and porphyries or occur as replace- 
ment bodies in the limestone beds. The ores are prevailingly pyritic, pyrite and pyirhotite being accompanied by 
galena, sphalerite, and silver-bearing sulphides, antimonides, and arsenides. Quartz, calcite, and barite are the 
ordinary gangue minerals. 

The ores were probably deposited shortly after the intrusion of the porphyries and before the uplift of the Mosquito 
Range by faulting. Thus two periods of faulting are recognized; the first dislocations provided the channels for ore 
deposition, the second series of faults displaced the ore bodies and effected the Mosquito uplift. 

Capps, S. R., and Lepfinqwell, E. D. K., Pleistocene geology of the Sawatch Range, near Lead ville, Colo.: Jour. 
Geology, vol. 12, 1904, pp. 698-706. 

Included in later publication by Capps, listed below. 

Wbstoate, Lewis G., The Twin Lakes glaciated area, Colorado: Jour. Geology, vol. 13, 1905, pp. 285-312. 

Describes the glacial geology of a part of the Lead ville region. Two epochs of glaciation are recognized and their 
deposits described. The terrace deposits of the upper Arkansas are ascribed to stream action. 

Emmons, S. F., and Irving, J. D., The Downtown district of Lead ville, Colo.: Bull. U. S. Geol. Survey No. 320, 1907. 

An advance portion of a complete revision of the Lead ville geology. The origin of the glacio-fluvial deposits is 
discussed at length and the presence of some true lake beds reafifirmed. The genesis of the Lead ville ores is reviewed 
in the light of past criticism and new knowledge, but the authors avowedly leave unanswered as yet some of the most 
important theoretical questions relating to the source of the ores. 

Van Horn, Frank R., A new occurrence of proustite and argentite: Am. Jour. Sci., 4th ser., vol. 25, 1908, pp. 

507-508. 

Describes and analyzes specimens of proustite and argentite from the California or Bell mine on Glacier Mountain, 
about 3 miles southwest of Montezuma. 

Occurrence of proustite and argentite at the California mine, near Montezuma, Colo.: Bull. Geol. Soc. America, 



vol. 19, 1908, pp. 9^-98. 

Practically the same as the foregoing paper. 

Lakes, Arthur, The general geology of Summit County, Colo.: Min. Sci., March 5, 1908, pp. 243-244. 
Brief notes on the general geology, veins, and placers. 

Spurr, Josiah E., Garret, George H., and Ball, Stdney H., Economic geology of the Georgetown quadrangle, 
together with the Empire district, Colorado [by Spurr and Garrey], with general geology [by Ball]: Prof. P:y^zc 
U. S. Geol. Survey No. 63, 1908. 

A very detailed account of a district only about 10 miles east of the Breckenridge district. The rocks are chiefly 
pre-Cambrian and are cut by pyritic gold-bearing veins and argentiferous galena-zincblende veins. The gangue of 



24 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COIX)RADO. 

both is largely quartz. The two classes of veins are described as genetically connected with two distinct groups of 
porphyritic intrusions. The auriferous veins are probably younger than the argentiferous veins. The original ores 
are believed to have been deposited from water of magmatic origin and to have undeigone considerable change and 
enrichment through the action of cold descending waters. The greater part of the gangue materials were derived 
from the wall rock. 

Patton, Horace B., The Montezuma mining district of Summit County, Colo.: First Ann. Rept. Colorado Geol. 
Survey, for 1908, Denver, 1909, pp. 105-144. 

Describes the general geology and divides the Archean rocks into the Idaho Springs formation, consisting chiefly of 
mica-sillimanite schists, and the '* hornblende gneiss series,'' comprising many varieties of foliated homblendic rocks. 
Three kinds of granite are distinguished and a number of intrusive porphyries and aplites are described imder the head- 
ing " Effusive rocks." * 

Two distinct vein systems are recognized, one set striking in a general northwest direction and the other striking 
between northeast and north. Most of the ore bodies are in the northeast-southwest veins. The dip is generally to 
the northwest and is steep, as a rule. The ores are described as being largely replacements of the country rock along 
fissures, but some veins have a crustified structure and are fissure fillings. The most abundant gangue material is 
quartz, but siderite and barite also occur in many veins. The ore minerals comprise proustite, stephanite, argentite, 
argentiferous galena, sphalerite, and chalcopyrite, of course with pyrite. The chief value of the ores in the past has 
been in their silver and gold contents. Secondary enrichment, according to Patton, has not been important. The 
veins are younger than the intrusive porphyries and may be of Tertiary age. 

Capps, Stephen R., jr., Pleistocene geology of the Leadville quadrangle, Colorado: Bull. U. S. Geol. Survey No. 
386, 1909. 

The glacial deposits described in this bulletin include those of the upper Blue and its tributaries to a point about 
a mile north of Breckenridge. Two epochs of glaciation are recognized. The older epoch is represented by small 
residual areas of decomposed morainal material and by the high terrace gravels of the valley of the Arkansas, which 
are interpreted as fluviatile deposits laid down by streams from the melting ice. The connection of these deposits 
with an older glacial epoch was long ago pointed out by Emmons (in the Leadville monograph), who, however, regarded 
the terrace gravels as having been deposited mainly in a lake. 

The younger epoch of glaciation has left abundant evidence of ice action in cirques, striations, moraines, and out- 
wash gravels. 

The areas near Breckenridge mapped by Capps as "drift of older epoch of glaciation" appear to include in part 
some of the later morainal material and in part some of the terrace gravels, no distinctly older drift having been detected 
in this particular locality during the present investigation. 

Bradford, A. H., and Curtis, R. P., Dredging at Breckenridge, Colo.: Min. and Sci. Press, Sept. 11, 1909, pp. 361-366. 
Useful as an account of technologic practice. 

1 The adjective "effusive" appears to be Inadvertently employed throughout the report for intrusive rocks, Patton stating that surface flovms 
are '^ entirely wanting." 



CHAPTER 11. 
PRE-CAMBRIAN AND SEDIMENTARY ROCKS. 

PRE-CAMBRIAN ROCKS. 
BEOIOXAL BELATIOXS. 

Granites, gneisses, and schists of pre-Cambrian age occupy extensive areas both to the 
east and to the west of the Breckenridge district (PI. I, in pocket). To the west they make 
up most of the Tenmile Range, and have been briefly described by S. F. Emmons in the Tenmile 
folio under the name ^*Archean." They extend to the southwest under the Paleozoic sediments 
of the Leadville district, and emerge to constitute the rocks of the Sawatch Range. To the east 
the same ancient crystalline terrane is exposed along the Front Range north of Moimt Guyot, 
and in the Georgetown quadrangle has been carefully studied by Ball,^ who distinguishes the 
Idaho Springs formation, consisting of biotite-sillimanite schist, biotite schist, and quartz gneiss, 
all originally sediments, from eight varieties of more or less metamorphosed igneous lock ranging 
from pegmatite and granite to diorite or dolerite. The Idaho Springs formation has been rec- 
ognized, also in the Montezuma district by Patton,^ who furthermore divides the *est of the 
local pre-Cambrian into a ** hornblende gneiss series," three different granites, and various 
rocks occurring as dikes. 

In that part of the Breckenridge district studied in preparation for the present report the 
pre-Cambrian rocks are visible at the surface only in a few small and for the most part outlying 
areas. The exposed masses, moreover, have been subjected to weathering that dates in part 
from Paleozoic time and as a rule are covered with soil and vegetation. In view of these con- 
ditions, to attempt in this area a subdivision of the pre-Cambrian and a thorough petrographic 
study of its constituent units would be a task so manifestly unprofitable that it was not under- 
taken. 

LOCAL DISTBIBTTTIOX. 

The largest area of pre-Cambrian rocks exposed in the Breckenridge district (PI. I) is in 
the northeast comer, at the head of Muggins Gulch. These rocks are known to extend some 
distance northeast of the area here mapped, and as Keystone Gulch,^ 8 miles northeast of 
Breckenridge, is in pre-Cambrian rocks, this terrane is probably continuously exposed from the 
head of Muggins Gulch to Montezuma and thence over the Continental Divide into the George- 
town, Idaho Springs, and Central City regions. 

Other areas of pre-Cambrian rocks are exposed along the western border of the district. 
These represent parts, slightly modified, of the old erosion surface upon which the Dakota 
formation was here deposited and they join farther west with the great pre-Cambrian mass of 
the Tenmile Range. Still another small but interesting exposure of pre-Cambrian rocks is that 
in the upper part of Illinois Gulch, in the vicinity of the Laurium and Moimtain Pride mines, 
where the sedimentary rocks have been eroded from the crown of a short irregular anticlinal 
fold, thereby exposing the crystalline complex upon which they were originally laid down. 

» Ball, S. II., Prof. Paper U. S. Oeol. Survey No. 63, 1908. 

* PattOD, H. B., The Montezuma mining district of Summit County, Colo.: First Ann. Rept. Colorado Geol. Survey, for 1908, 1909, pp^ 
105-144. 

* Called Buflalo Qulch on the Hayden map, Keystone Gulch there being the next one west. 

25 



26 GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

PETBOOBAPHT. 

The most abundant variety of pre-Cambrian rock in the Breckenridge district is a gray fissile 
micaceous schist composed essentially of quartz, biotite, and muscovite, the last in the fine- 
leaved form known as sericite. This is probably the equivalent of the Idaho Springs formation* 
of the Georgetown and Montezuma districts. Another fine-grained thinly foliated rock prevalent 
about the head of the Blue and elsewhere in the Tenmile Range, as well as near the Swandyke, 
at the head of the Middle Swan, east of Breckenridge, is less fissile than the rock just mentioned 
and has a gneissic rather than a schistose structure. The microscope shows this rock to consist 
of quartz, biotite, muscovite, microcUne, and a little plagioclase. 

At the mouth of North Barton Gulch, 2^ miles north of Breckenridge, is a dark biotitic 
gneiss, a little coarser in texture than those just described, which is a foliated granite approach- 
ing quartz monzonite in composition. Another fine-grained, beautifully foliated, dark-gray 
gneiss, prevalent about the head of St. Johns Gulch between Breckenridge and Montezuma, 
contains hornblende as its principal dark constituent and is a quartz diorite gneiss. 

All the fine-grained schists and gneisses are intricately and irregularly intruded by various 
kinds of granitic and pegmatitic rocks. Within that part of the district specially studied the 
most abtmdant pre-Cambrian intrusive rock is a medium-grained (about 5 millimeters) reddish 
granite, of which the conspicuous mineral constituents are flesh-colored feldspar, quartz, and 
a subordinate quantity of muscovite. The microscope shows that the feldspar includes ortho- 
clase and microcline, both generally microperthitic. Other intrusive rocks, noted particularly 
at the head of the Blue and at Swandyke, are nearly white pegmatites consisting of orthoclase, 
microcline, quartz, and muscovite with magnetite locally abundant in small irregular bunches. 
The pegmatites vary in composition from those with little quartz to those that are chiefly 
quartz and^muscovite. 

Thorough study of a large well-exposed body of the pre-Cambrian rocks, such as that 
fo rmin g the mass of the Tenmile Range, would doubtless discover many more varieties of 
gneiss, schist, and pegmatite than those here mentioned. The brief descriptions given, how- 
ever, will suffice to indicate in a general way the character of the old foundation upon which 
the younger sedimentary rocks of the Breckenridge district rest 

STRATIFIED ROCKS. 
GENERAL STBATIOBAPHY OF THE REGION. 

The part of central Colorado that is adjacent to the Breckehridge district exhibits a rather 
full and unusually uniform succession of beds from the Cambrian to the Cretaceous. Tertiary 
strata also, notably the famous insect-bearing lake beds near Florissant, occur in South Park, 
50 miles southeast of Breckenridge, but these are local deposits that never extended into the 
area with which this report deals. 

To attempt here any extensive treatment of the stratigraphy of central Colorado would be 
inappropriate; it is suflicient to present such saUent facts as have direct bearing on the problems 
of the Breckenridge district. G. H. Girty, in his exhaustive monograph on the Carboniferous 
of Colorado,* has summarized and fully discussed the literature, not only of the Carboniferous, 
but virtually of the whole Paleozoic of the State. To his work those who wish to pursue the sub- 
ject further may be referred. 

The Breckenridge region marches with the Eobinson or Tenmile district to the west and 
with the LeadviUe district to the southwest. Fortunately the stratigraphy of these two dis- 
tricts has been carefully studied and the results are available for comparison. The stratigraphic 
column at Aspen, 45 miles west-southwest of Breckenridge, has also been accurately described 
and less satisfactory data have been recorded for Red CUff, 20 miles northwest of Breckenridge. 
The essential stratigraphic results of these investigations are summarized in tabular form on 
the insert facing this page. The column in the table under **Red Cliff involves some inter- 
pretation of Tilden's section, which in its actual published form is as follows: 

» Prof. Paper U. S. Oeol. Survey No. 16, 1903. 



jliin 



•m 



m 

ill 



1 



I a 



y. 



i ' I 






I 



I 

I 
1.1 

IM 



li 
I 



mn 



If 



yi 

I 
I 



1:1 



I 



I 



HJi 



I 



i 

I 

I it 



I 

I 

If 
II 



I I 



li: 

I 



I 



4f' 

i 



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il 

I 



d 1 .2« ; 



PBE-CAMBRIAN AND SEDIMENTABY ROCKS. 27 

« 

Gneiss. Feet. 

Cambrian white quartzite : 126 

Fine-grained sandstone 100 

Silurian white quartzite 5 

Conglomerated quartzite 80 

Blue dolomitic limestone 250 

Black siliceous limestone 300 

Carboniferous limestone 1, 000 

Leadville quartz porphyry 200 

Concerning this Girty remarks: 

Probably the four lower members of the section, aggregating 238 feet, should be considered as representing 
the Sawatch quartzite * * *. The next two divisions, amounting to 550 feet, have the position and character of 
the Leadville limestone. If they be taken to represent the Leadville, however, no equivalent of the Yule limestone 
and Parting quartzite can be found. It is clear from Emmons's remarks that these formations are not normal at this 
locality. If a portion of the blue dolomitic limestone belongs to the Yule, the Parting quartzite would still be missing. 
Tilden seems to suggest that the third member of the section, which he designates the Silurian quartzite, is the Parting 
formation. If this is indeed the case the Yule limestone as such is absent. The next bed, consisting of 1,000 feet of 
limestone, would appear to represent the Weber formation of the Aspen district and the Weber shale of the Tenmile 
district. 

The ages of some of the formations listed in the table have been satisfactorily determined 
on the evidence of fossils, this being generally true of the beds from the Maroon formation down- 
ward, with the exception of the "Parting'' quartzite; but the age and correlation of the series 
of red beds lying between the Maroon and Dakota formations are still imperfectly known. 
Thus the ** Wyoming" formation of the table has never been shown to correspond accurately 
to the formation in the Denver Basin to which the name was originally appUed, although most 
geologists who have studied the beds both in central Colorado and on the east flank of the Front 
Range agree in considering them practically equivalent.* If this supposition is correct, then of 
course the Fountain formation of the east slope is equivalent, in part at least, to the Maroon 
formation west of the Front Range and similarly the Morrison formation, overlying the '* Wyo- 
ming '^ in the type locaUty of the latter, is equivalent to the upper part of the Gunnison forma- 
tion or to the McElmo formation farther west. The bright-red beds of the ''Wyoming" have 
generally been considered Triassic, but the possibility of their being wholly or partly Permian 
has never been entirely eliminated. Emmons * records that L^es found, just east of Fairplay, 
in beds above what was later named the Maroon formation, plant remains determined by Les- 
quereux as Permian, and fossil insects determined by A. Hyatt as Triassic. Darton,^ also, 
presents evidence indicative of Permian age for at least the lower third of the Upper Wyoming 
of Eldridge in the Denver region and Cross and Howe* assign "the entire red-bed sections of 
the Denver region and southward, at least to the Canyon City embayment, to the Paleozoic." 
The age of the Morrison formation, according to Stanton,* still awaits accurate determination. 
Whether it is Jurassic or Cretaceous (Comanche) is unknown. 

HOOSIEB PASS SECTION. 

One of the most instructive sections in the region, preparatory to a study of the stratig- 
raphy of the Breckenridge district, is that across Hoosier Pass (see PI. Ill, p. 16), which has 
been described in part by S. F. Emmons." Lying 8 miles due south of Breckenridge, this section 
aflfords an opportunity of examining the entire stratigraphic sequence from the pre-Cambrian 
of North Star Mountain (Star Mountain or North Peak on some maps) on the west to the 
black Upper Cretaceous shale at Breckenridge Pass or Boreas on the east. The general strike 
of these beds ranges from north to N. 25° W.; the dip, except at one place in Hoosier Pass, 

» See Olrty, O. H., op. clt., p. 190. 

* Geology and miniag Industry of Leadville: Mon. U. 8. Geol. Survey, vol. 12, 1886, p. 71. 

s Darton, N. H., Comparison of the stratigraphy of the Black Hills, Bighorn Mountains, and Rocky Mountain Front Hange: Bull. Geol. Soc. 
America, vol. 15, 1904, p. 416; Geology and underground water resources of the central Great Plains: Prof. Paper U. S. Geol. Survey No. 32, 1905, 

pp. 81-87. 

* Cross, Whitman, and Ilowe, Ernest, Red beds of southwestern Colorado and their correlation: Bull. Geol. Soc. America, vol. 16, 1905, p. 492. 
( Stanton, T. W., The Morrison formation and its relations with the Comanche series and the Dakota formation: Jour. Geolog>% vol. 13, 1905. 

p. 668. 

* Geology and mining industry of Leadville, Colo.: Mon. U. S. Geol. Survey, vol. 12, 1886, pp. 100-104, atlas sheets VI and VIII. 



28 GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

where, as Emmons shows, there is a local synclme, is uniformly east at angles ranging from 20° 
to 45°, the average or general dip being about 35°. The distance nearly across the general 
strike of the beds from the base of the section west of Hoosier Pass to Boreas is approximately 
8 miles. As Emmons has remarked,* the section does not lend itself to accurate measurements 
of total thickness. The best exposures are along the winding Continental Divide, which in 
places makes small angles with the strike. Even on this ridge the exposures are not continuous 
and it is impossible to be sure that no duplication has been eflFected by faulting. The sediments, 
moreover, are intruded by numerous bodies of porphyry, both as sheets and dikes, and the 
extent to which these intrusions have increased the apparent thickness of the sedimentary rocks 
can be determined only when the geology on both sides of the line of section has been accurately 
mapped. Notwitlistanding these deficiencies, the section is an excellent one along which to 
study the varied lithology of the successive formations. 

North Star Mountain, upon which the base of the section rests west of Hoosier Pass, is a 
narrow spur of gneiss and granite jutting eastward from the crest of the Tenmile Range between 
Quandary Peak on the north and Mount Lincoln on the south. At the east end of its nearly 
level crest, at an elevation of 13,400 feet, the spur is capped by Cambrian quartzite (Sawatch 
quartzite of Emmons) dipping gently eastward. This is about 100 feet thick and, at the few 
places where the contact with the pre-Cambrian was clearly examined, is not accompanied by 
any basal conglomerate. The weathered rock is nearly ^hite, but specimens from shafts are 
greenish gray, the green tint being due to a little chlorite in the quartz that cements the original 
sand grains. 

Overlying the quartzite on the crest of the spur is an isolated mass of Cambrian limestone 
(Yule limestone of Emmons) not shown in Emmons's map or section. East of this limestone 
the slope leading down to Hoosier Pass for a distance of nearly 4,000 feet is virtually a dip slope 
on the quartzite from which the limestone has been eroded. Then the limestone reappears, 
dipping 30° E. Emmons's section shows both the Yule and Leadville limestones with a total 
thickness of about 400 feet and he describes the *'Blue'' or Leadville limestone as '*a dark 
iron-stained dolomite, weathering black." In a necessarily hasty trip over this part of the sec- 
tion in 1908, 1 failed to find any exposure of the ** Parting'' quartzite or to distinguish any essen- 
tial difference between the upper and lower beds of limestone. The prevailing variety is a sandy 
dolomite, pinkish gray on fresh fracture, but weathering buff or rusty brown. Much of it is 
banded, and on weathered surfaces the bands stand out as narrow, parallel rust-brown ridges of 
porous material consisting mainly of silica and limonite. Possibly one thoroughly f amihar with 
the Leadville section might be able to distinguish the two limestones west of Hoosier Pass; but 
in the absence of such special knowledge most observers would probably see at this locality 
little suggestive of a division of the limestone series. 

Overlying the limestone are thin-bedded gray and buff arkosic grits and conglomerates 
with intercalated beds of dark micaceous shale in which Emmons found ** casts of Zaphrentis 
and corals."^ These are the '* Weber shales" of the Leadville monograph. At this locality, 
however, arkosic grits are fully as abundant as shales and there is certainly no distinctively shaly 
division at the base of the ^^Weber formation." The prevalent beds of the **Weber," all the 
way down the slope to Hoosier Pass and up to an elevation of about 12,800 feet on the slope 
east of the pass, are gray, very micaceous, quartzose grits. Some are fine grained and shaly, 
looking almost Uke mica schists. Others are conglomeratic with pebbles of well-rounded white 
quartz. Some beds are mostly quartz and muscovite; others are granitic arkoses, contain 
abundant microchne with some orthoclase, and weather to a soil that, were the arkosic character 
of the terrane unknown, would strongly suggest underlying granite. No limestone was noted 
in the *' Weber" of tliis section except one tbin bed near the top of the formation. The whole 
thus contrasts strongly with the calcareous ** Weber formation" of the Aspen district and was 
evidently deposited in shallow water close to a pre-Cambrian land mass that furnished the 

1 Op. cit., p. 101. * Idem, p. 100. 



PRE-CAMBBIAN AND SEDIMENTARY ROCKS. 29 

abundant quartz sand and flakes of muscovite. Emmons's section assigns a thickness of 2,000 
feet to the ^* Weber" at Hoosier Pass, but this is avowedly an estimate, not a measurement. 

Above the ** Weber" in Hoosier Ridge, east of the pass, come the *^ Upper Coal Measures" 
of the Leadville report, or the beds now known as the Maroon formation. Lithologically these 
resemble the ^* Weber" in all but color. The ^' Weber formation" is invariably gray; the 
Maroon formation is generally dark red, but in detail shows many alternations of gray and 
red or variegated beds. The Maroon appears to be perfectly conformable with the "Weber,*' 
and there is nothing in this section to mark a definite boundary between the ''Weber" and 
the Maroon unless it is the appearance of the first reddish bed. Emmons refers to ''a bed of 
dark-blue limestone about a hundred feet in thickness" in the lower part of the Maroon, from 
which he obtained a number of Pennsylvanian fossils.* What is supposed to be this same lime- 
stone was crossed in my own traverse of 1908 at an elevation about 75 feet higher than the 
lowest of the dark red beds. As locally exposed, however, it did not appear to be over 15 feet 
thick and yielded no fossils. Another thin bed of similar limestone outcrops in the "Weber" 
about 75 feet below the lowest red bed. 

Two miles east of Hoosier Pass the Continental Divide turns north along the crest of Hoosier 
Eidge toward Red Peak, the line of traverse here being nearly parallel with the strike for about 
2 miles. This turn marks the east end of Emmons's section, the crest of the ridge being here 
formed by a sheet of porphyry. 

Near Red Peak, which apparently consists mainly of eastward-dipping Maroon beds capped 
by a thick sheet of porphyry, the divide turns northeast, south of Horseshoe Basin, toward 
Breckenridge Pass. Here the traverse showed a gradual transition from the alternating gray 
and dark-red beds of the Maroon formation into the brilliant brick-red sediments of the "Wyo- 
ming" formation, consisting of generally rather thin bedded micaceous shales, flaggy ripple- 
marked sandstones, and cross-bedded conglomerates, all bright red except where metamor^ 
phosed by porphyry intrusions into greenish-gray epidotized rocks. The red pigment of these 
sediments is mainly in the more or less calcareous cement that holds together the grains of 
quartz and feldspar and the very abundant muscovite flakes 

The pebbles in the "Wyoming" are chiefly quartz, are not well rounded, and are rather 
sporadically distributed through their sandy matrix. In fact, the coarsest beds are more 
aptly described as pebbly grits or pebbly sandstones than as conglomerates. With the quartz 
pebbles in some beds are pebbles of various pre-Cambrian crystalline rocks. Thin beds of gray 
limestone are fairly conmiion in the "Wyoming." A characteristic feature of the red beds, both 
in this formation and in the Maroon, is the presence of greenish spots due to local bleaching of 
the iron-oxide coloring material by natural reducing agents. 

The thickness of the "Wyoming" as exposed between Red Peak and Boreas can not be 
accurately estimated from a single traverse, owing to the complications introduced by the 
porphyry intrusions and to some uncertainty as to the exact position of the base of the "Wyo- 
ming" or the top of the Maroon. The width of the belt nearly at right angles with the gen- 
eral strike is about 2 miles. This, with an average dip of 35°, would give a thickness of about 
6,000 feet. From this something must be subtracted for the included porphyry sheets, which, 
however, in this part of the section are probably less than 1,000 feet in aggregate thickness. 
There is little suggestion of important faulting, and it appears reasonable to conclude that 
5,000 feet is a fair estimate for the total thickness of the "Wyoming" in tliis section. Emmons 
gives the maximum thickness in the Tenmile district at 1,500 feet, but the formation has there 
suffered some loss by erosion. 

If the Morrison formation is represented in the Hoosier Pass section it has not been dis- 
tinguished from the "Wyoming," which about three-quarters of a mile west of Boreas appears 
to be conformably overlain by what in this region has generally been considered, since the days 
of the Hayden Survey, as the Dakota formation. This is well exposed on a little knoll on 
the ridge, striking N. 16° W. and dipping 43° E. At the base is a bed about 6 feet thick of 

.1 Op. cit., p. 103. 



30 GEOLOGY AND ORE DEPOSITS OF BBECKENBIDGE DISTBICT, COLORADO. 

coarse light-buff pebbly sandstone. The pebbles, which are abundant, are of white quartz and 
are mostly under half an inch in diameter, but range from the smaller sizes up to those 2 inches 
across. This pebbly bed is succeeded by buff sandstones in beds of similar moderate thickness, 
the whole constituting a basal arenaceous member about 50 feet thick. Above this comes a 
middle thin-bedded member about 100 feet thick consisting of thin argillaceous and arenaceous 
limestones with some pebbly grits and a little dark shale. These weather pink or light red as 
a whole, but are not so red as the *' Wyoming*' beds. Some porphyry is irregularly intruded 
into these thin beds west of Boreas. Overlying this middle member is an upper sandy division 
consisting of 50 feet of thick-bedded, even-grained buff sandstones. This is the most con- 
spicuous member of the Dakota and to the north changes to a hard white quartzite that resists 
erosion and forms the sunmiits of many of the hills near Breckenridge. Although the Dakota 
formation as a whole is persistent over vast areas, it is apparently variable in the character and 
succession of its beds. In some places, as in the vicinity of Dillon, north of Breckenridge, the 
Dakota shows three prominent sandstone members, and other variations will appear when the 
rocks of the Breckenridge district are described in detail. 

The beds overlying the Dakota and occupying Breckenridge Pass are very poorly exposed. 
In some places a little fine-grained gray sandstone is visible, and a short distance above the 
Dakota are three or four beds of gray limestone, up to 4 feet in thickness, interbedded with the 
sandstone. But probably most of the rock under the grass-covered slopes of the pass is a 
dark fissile shale, which will be described later from other and better exposures than are afforded 
by this particular section. This shale is intruded just east of Boreas by the great porphyry sheet 
of Bald Mountain, and there is no way, either on the line of the Hoosier Pass section or else- 
where in the vicinity of Breckenridge, of ascertaining its total original thickness. On Trout 
Creek, 4 miles east of Fairplay and 13 miles south of Boreas, Peale's section ^ indicates the 
possibility of these shales being over 1,000 feet thick. As will be later shown, the black and 
gray shales of this region carry fossils of Benton, Niobrara, and possibly also of Montana age, 
and, as it is impracticable to divide these rocks into distinct formations corresponding to these 
chronologic divisions of the Cretaceous, the whole will be referred to as the Upper Cretaceous 
shale. 

It will appear from the foregoing notes on the Hoosier Pass section that in the area from 6 
to 8 miles south of Breckenridge the Dakota is separated from the pre-Cambrian by several thou- 
sand, perhaps 10,000, feet of sediments whose beds strike in a general way toward Brecken- 
ridge. Two miles north of that town the Dakota, on the other hand, rests directly upon the 
pre-Cambrian. Thus within a distance of 10 miles the entire stratigraphic section of the Lead- 
ville and Tenmile districts has disappeared. This fact was indicated in a general way on the 
Hayden map of Colorado, which, however, was probably nowhere more inaccurate than in the 
part pertaining to the Breckenridge and Leadville region. Some explanation of this trans- 
gression of the Dakota upon the pre-Cambrian will be attempted in a later chapter. 

Although there is no apparent unconformity in the . Hoosier Pass section, it should be 
remembered that Eldridge,* in the Crested Butte region, found the Gunnison formation resting 
unconfonnably on the Maroon formation and that Cross and Howe,* at Ouray and elsewhere, 
have recognized another important unconformity at the base of the Triassic Dolores formation* 

SEDIMBNTABT FORMATIONS OF THE BBECKENKIDOE DISTBICT. 

FORMATIONS REPRESENTED. 

The sedimentary formations occurring in the mapped part of the Breckenridge district 
(PL I, in pocket) are the ''Wyoming" formation, the Dakota sandstone, and the Upper Creta- 
ceous shale. As is apparent from the geologic map, the smallness of the area and the great 

1 Seventh Ann. Rept. U. S. Qeog. and Qeol. Survey Terr., for 1873, 1874, p. 218. 
* Eldrldge, G. H., Anthracite-Crested Butte foUo (No. 9), Qeol. Atlas U.S., U. 8. Qeol. Survey, 1894. 

s Cross, Whitman, and Howe, Ernest, Red beds of southwestern Colorado and their correlation: Bull. Qeol. Soc. America, vol. 16, 1905, pp. 
447-408. Cross, Whitman, Stratigraphic results of a reconnaissance in western Colorado and Utah: Jour. Geology, vol. 15, 1907, pp. 634-679. 



PBE-CAMBRIAN AND SEDIMENTAEY ROCKS. 31 

• 

local disturbance of the beds by the intrusion of many irregular masses of porphyry render the 
Breckenridge district a field that is exceptionally unfavorable for the study of the sedimentary 
rocks and not likely to add greatly to existing knowledge of the regional stratigraphy except as 
regards the interesting problem raised by the disappearance of the Paleozoic beds between 
Breckenridge and Hoosier Pass. Even this problem might be much more readily and convinc- 
ingly solved could accurate topographic and geologic mapping have been carried southward to 
the head of the Blue. 

''WYOMING" FORMATION. 
XTAME. 

The name "Wyoming" might not have been applied in this report to the generally bright- 
red beds underlying the Dakota had not Emmons previously used it in the adjoining Tenmile 
district. The correlation of the red beds of the Tenmile district with the "Wyoming" of the 
Denver Basin having been authoritatively made, however, it is better to accept it than to intro- 
duce any change into the nomenclature, particularly as the detailed study of the small Brecken- 
ridge area, in which the formation is not fully and typically developed, is a wholly inadequate 
foundation upon which to base any essential modification of current terminology. The "Wyo- 
ming" formation of the Breckenridge district is probably the stratigraphic equivalent of the 
Lykins formation of the Boulder district, Colorado. As Cross and Howe ^ have remarked, it is 
premature to attempt direct correlation of the Dolores (Triassic) and Cutler (Permian) forma- 
tions of southwestern Colorado, as such, with the stratigraphic units of central and northern 
Colorado. 

DISTRIBXrTIOXr. 

The "Wyoming" formation is displayed mainly in the southern part of the district, the 
only beds north of Breckenridge referable to this formation being some that are exposed in placer 
pits a^d tunnels along the east side of the Blue, between the mouth of French Creek and Brad- 
docks station. The largest area shown on Plate I (in pocket) is that adjacent to the Laurium 
and Mountain Pride mines, extending from the head of Australia Gulch on the north to a point 
a short distance south of the mapped area. This body of sediments occupies the greater part of 
the basin drained through Illinois Gulch and may be conveniently referred to as the Illinois 
Gulch area. 

Not entirely separate from this area is another, which is drained in large part by Indiana 
Gulch and, south of the district proper, by Pennsylvania Gulch. Part of this area is crossed by 
the railway between Boreas and Bacon, the numerous cuts affording excellent exposures of the 
vivid red grits. It is the rock at Famham station, between Bacon and Boreas, and of the War- 
rior^s Mark mine, south of that station. Characteristic exposures may be examined also along 
the wagon road up Indiana Gulch to Boreas and all the way up Pennsylvania Gulch to the fine 
amphitheater at its head known as Horseshoe Basin. The same formation also probably under- 
lies a large part of the morainal material and glacial silt south of Breckenridge. 



The thickness of the "Wyoming" formation at the south edge of the Breckenridge area, 
near Bacon, can not be accurately measured, but is probably at least 1,000 feet. It is estimated 
that a little farther north, east of the Laurium mine, from 600 to 700 feet of these beds intervenes 
between the pre-Cambrian and the Dakota. Thence the formation thins rapidly to the north. 
Along the west base of Gibson Hill the beds are so poorly exposed that no reliable estimate of 
their thickness is possible, but the maximum can scarcely exceed 400 feet and, inasmuch as the 
Dakota in the neighborhood of the tliree Barton gulches rests on the pre-Cambrian, is probably 
much less than that. The apparent thickness here is perhaps due to faulting. 



LITHOLOOT. 



In Indiana Gulch the "Wyoming" beds are uniformly bright red and are for the most part 
rather coarse quartzose grits, many of them pebbly, grading upward and downward into flaggy 

- Red beds of southwestern Uolorado and their correlation: Bull. Qeol. Soc. America, vol. IG, 1905, p. 488. 



32 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

micaceous sandstones interbedded with sandy, micaceous red shales. The coarser material in 
some places shows cross-bedding, and many of the shale layers are ripple marked. The imper- 
fectly rounded pebbles are as a rule of white vein quartz, although some beds contain fragments 
of crystalline pre-Cambrian rock. The pebbles occur as ill-defined bands or streaks in the 
laminated grits and not as definite persistent beds of conglomerate. The different beds vary in 
the character of their cementing material. Some effervesce freely with acid, showing that the 
cement is mainly calcite. Others show little or no effervescence and are probably rendered 
coherent by films of quartz or oxide of iron. 

A few thin lenticular beds of compact gray limestone occur in the grits and shales. One 
of these, about 1 foot thick, is exposed on the slope 2,200 feet east of Bacon. At least two similar 
beds outcrop on the north side of lUinois Gulch, west of the pre-Cambrian area. 

As the red beds are followed northward to the vicinity of the pre-Cambrian area in Illinois 
Gulch they change in character. The intensely red color so characteristic of the formation 
farther south becomes less noticeable and the sediments assume a variegated aspect. Many of 
the beds, especially the shaly ones, retain their bright hue but others are gray, buff, or maroon. 
In short, the formation as it approaches the pre-Cambrian exhibits features strongly suggestive 
of the Maroon and '^ Weber" formations. A diamond-drill hole bored some years ago at the 
Mountain Pride mine should supply much interesting information as to the character and 
thickness of the formation, but no record of the material passed through has been found. The 
pieces of core scattered about the ground at the abandoned mine consist of arkosic grits with 
much pink orthoclase or microcUne and abundant muscovite. Some are coarse and pebbly; 
others are fine grained and in consequence of the original settling of the mica fiakes on their flat 
sides can be spUt with eisise in planes parallel with the bedding. A large part of the core is gray, 
but some of the fine flaggy micaceous sandstones are striped with dull maroon and some finely 
micaceous calcareous shale is Ught pinkish red. Shale identical with the last-named variety 
occurs in the Crold Bell tunnel, 1 mile a Uttle east of north from the Mountain Pride mine, and 
in the French Creek tunnel, half a mile southwest of Lincoln. In both of these tunnels this 
pink or red shale occurs in the upper part of the formation and is underlain by arkosic grits. 
These grits, varyingly pebbly, as exposed in superficial openings between the Gold Bell tunnel 
and the pre-Cambrian inher are mainly gray but include also some buff, maroon, and red beds. 
In the French Creek tunnel, which cuts the formation about 500 feet below the surface and 
under the Bald Mountain sheet of diorite porphyry, the grits are all gray and contain finely 
disseminated pyrite. Some of the coarser bands in the arkosic material contain angular frag- 
ments of fresh feldspar up to IJ inches in length, muscovite flakes half an inch broad, and quartz 
pebbles up to 2 inches in diameter. 

It thus appears that the sediments between the pre-Cambrian and the Dakota in the 
Breckenridge district are not distinctly of "Wyoming" type but are in part identical in litho- 
logic character with beds found in the Maroon and '^ Weber" formations. 

CAUSE OF THE RED COLOR. 

Study of thin sections of the red sandstones shows that their color is due to thin films of 
ferric oxide coating the surfaces of the detrital grains and not to any original redness of the 
constituent particles. Hard, dense grains, like those of quartz or fresh feldspar, have merely a 
skin of red pigment. Porous, decomposed, or lamellar minerals have had the iron oxide deposited 
within them in microscopic pores or along planes of parting. Some of the red-coated grains are 
aurrounded by calcite, and it is difficult to avoid the conclusion that the sediments were red before 
they were fuUy cemented. On the other hand, the coloring matter clearly was not introduced 
until after attrition of the grains had ended and they had come to rest in the beds where they 
now lie. 

The cause of the red color of some sandstones and shales has been investigated by many. 
Among the later contributions to the subject is the work of G. B. Richardson,^ who concluded 
that the dominant factor was the derivation of the beds from a residual red soil, although the 

1 The upper red beds of the Black Hills: Jour. Geology, vol. 11, 1903, pp. 3S1-393. 



PBE-CAMBBIAN AND SEDIMENTARY BOCKS. 33 

dehydrating effect of deposition in salt water, as suggested by W. Spring, is given some weight. 
More recently Joseph Barrell ^ has summarized in part the literature on this problem and, in 
general agreement with W. O. Crosby, holds that the redness is normally assumed on consoli- 
dation by any sediment containing an appreciable quantity of ferric hydrate, through dehydra- 
tion favored by several factors, among which he mentions time, pressure, high temperature, and 
alternating wetness and dryness but does not refer to the probably important factor of salinity. 

The red color of the "Wyoming" formation near Breckenridge can not be wholly the effect 
of atmospheric agencies on the beds as now exposed. The red beds are just as ruddy in rail- 
way cuts and in many outlying prospect shafts as at the smrface. The deposition of inter- 
stitial calcite over the red films also points to the existence of the color prior to the present 
cycle of weathering. Moreover, Mr. N. H. Darton informs me that east of the Rocky Momi- 
tains deep wells in the plains, penetrating the red beds far from their outcrop, show that the 
color persists far below the present reach of weathering. 

On the other hand, some mine workings show a change from red sediments to gray, and 
Enmions,' in describing the Maroon formation in the Tenmile district, states that in depth, 
as shown in miderground workings, the red color is generally replaced by greenish gray, imply- 
ing that the red hue is due to weathering of the present beds. The Breckenridge district is 
not a favorable one in which to pursue exhaustively the study of the colors of sedimentary 
rocks, but such evidence as is available indicates that the color of the red beds is not due to 
original redness of the imconsolidated sediments and is only in small part due to recent weath- 
ering of exposed beds. The ferric oxide pigment may locally be reduced by organic or sulphide 
solutions. Where pyrite has formed in the rocks, as in the French Creek tunnel, the red color 
has been obliterated, and inasmuch as disseminated pyrite is abundant in the vicinity of most 
ore bodies in these beds, it is probable that the change of color noted in the Tenmile district 
by Emmons, and to some extent observable in the Breckenridge district, does not mean that 
the rocks deep underground have never been red, but does show the bleaching effect of the 
formation of pyrite. It is doubtful whether there is any depth at which the strongly colored 
*' Wyoming" beds change generally to gray independently of the presence of pyrite. 

CORBZLATZOV. 

No fossils have been found in the "Wyoming" formation of the Breckenridge area. The 
lithologic character of the beds raises the question whether they should be regarded as repre- 
senting the thin edges of the "Weber," Maroon, and "Wyoming" formations, or whether they 
belong solely to the last, as is indicated by their general strike. It is difficult, however, in 
view of the thickness of the "Wyoming" in the apparently conformable Hoosier Pass section, 
to imagine conditions under which any deposits of "Weber" or Maroon age could have been 
laid down in the Breckenridge district. That area was probably land at the time these two 
formations accimiulated and supplied part of the detritus of which they are composed. All 
three formations are clearly of shaUow-water origin and were laid down at no great distance 
from shore. As the land slowly sank sedimentation kept pace with the movement, and the 
near-shore or littoral conditions under which the base of the "Weber" first overlapped the 
pre-Cambrian were gradually carried upward in the chronologic scale imtil they reached 
"Wyoming" (Triassic?) time. In any sedimentary basin gradually filled by long-continued 
subsidence of the land a vertical section, such as is given by a deep bore hole made at a point 
far out from the later shore line of the basin, will normally show a certain definite sequence 
of beds, perhaps readily divisible into many formations belonging to different geologic periods. 
Borings made successively nearer to the edge of the basin might be expected to show some of 
the formations disappearing, becoming coarser, merging with others above or below, and, in 
general, showing increasing irregularity in consequence of local variations in currents and 
sediments close to the shore line. Finally, close to the edge, or latest shore, all the older forma- 
tions will have disappeared, and perhaps the coarse shingle last formed by the waves closely 

1 Relations between climate and terrestrial deposits: Jour. Geology, vol. 16, 1908, pp. 285-294. 
• Tenmile district special folio (No. 48), Oeol. AUas U. S., U. S. Geol. Survey, 1898, p. 2. 

90047"— No. 75—11 3 



34 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

resembles and is actually continuous with the much older basal conglomerate found at the 
bottom of the basin in the deepest boring. The hypothetical conditions thus outlined are, 
it is believed, applicable in a general way to the explanation of the hthologic change in the 
"Wyoming" formation as it thins out northward and particularly to the fact that, where 
deposited directly on the pre-Cambrian, its basal portion resembles the grits and conglomerates 
of the much older "Weber formation." 

The Hayden geologic map of north-central Colorado shows a strip of "Jurassic (?), vari- 
egated beds, etc.," extending from Hamilton through Breckenridge Pass to French Creek 
and lying between the "Triassic " red beds and the Dakota. A. C. Peale,^ however, who studied 
the geology of the South Park division, describes the red beds of Tarryall Creek, a short distance 
above Hamilton, as Triassic, and mentions the Dakota ("Cretaceous No. 1") as directly over- 
lying them. In his section No. 9, made from Platte River eastward to Trout Creek, about 
5 miles north of Fairplay, Peale, traveling east and from lower to higher beds, gives the following: 

52. Space, the valley of Crooked Creek. On the east side of the valley we have the mas- 
sive red sandstones (Triassic ?) with all the characteristics of the same beds east of 
the foothills, and on Trout Creek west of them. It is probable they extend down to 
bed 51. Their softness has allowed them to be worn down, and they have been Feet, 

covered with debris. Total thickness f, 300-1, 500 

63. Coarse pink sandstones 25 

63. Fine-grained rose-colored sandstones 5 

These two beds are the upper part of the red beds. 

64. Rather coarse ciEdcareous sandstones, shales mottled red and gray 5 

65. Space, probably filled with continuation of 54, grading into. the next bed 30 

66. Gray compact limestone. This limestone has cross cleavage, and becomes harder as 

we go up 10 

67. Hard fine-grained limestone, light gray 15 

68. Space, probably filled with limestones and shales 75 

59. Outcrop green shales 10 

60. Space, filled with shales and sandstones 60 

61. Rusty yellow sandstone 5 

In the foregoing, which is a. small part only of the full section, the beds numbered from 
54 to 60, inclusive, aggregating 205 feet in thickness, correspond to the variegated Jurassic 
beds of the Hayden Survey and perhaps to the Morrison formation of later writers. No. 61 
in the foregoing section Peale regarded as the basal bed of the Dakota and No. 53 as the 
upper bed of the Hayden Triassic or what has since been named the "Wyoming" formation. 

What is now known as the Breckenridge district appears to have been between the area 
studied by Marvine on the north and that investigated by Peale on the south, and the distri- 
bution of the rocks near Breckenridge as represented on the Hayden map was evidently more 
the result of inference than of observation. The strip of "Juratrias" extending through to 
French Creek was perhaps surmised mainly from Peale's section, quoted in part on this page. 
In view of the absence of fossils, the poor exposures, and the disturbing effect of igneous intru- 
sions in the Breckenridge area it is not safe to assert that beds equivalent to the ilorrison 
formation are wholly lacking. It has not been found possible, however, to distinguish and 
map any formation between the Dakota and the "Wyoming.'' 

DAKOTA SANDSTONE. 
DIBTRIBT7TI0N. 

The beds referable to the Dakota are exposed in an interrupted belt stretching from the 
middle of the southern border of the area to its northwest corner. South of the area mapped 
in this paper the Dakota outcrops continue past Boreas to Hamilton, and are shown by the 
Hayden map along the western edge of South Park. To the north the same map indicates 

» Seventh Ann. Rept. Oeol. and Geog. Survey Terr., for 1873, 1874, pp. 214, 216. 



PBE-CAMBRIAN AND SEDIMENTARY ROCKS. 35 

the general continuai^ce of the Dakota belt along the Blue to its confluence with the Grand 
and far beyond the latter river into North Park. It also represents the formation as crossing 
the Park Range 30 miles northwest of Breckenridge and connecting directly with broad areas 
of the Dakota spread over hundreds of square miles of plateau country in western Colorado.* 
From the Dakota of the Great Plains the formation at Breckenridge is separated by the pre- 
Cambrian axial belt of the Front Range, the distance from Breckenridge eastward to Morrison 
being about 45 miles. 

Within the narrow limits of the Breckenridge district the Dakota has been so broken by 
igneous intrusion and so dissected by erosion as to be Uttle more than a chain of fragments. 
One very irregular area stretches from the south edge of the district across Illinois Gulch to 
Nigger Hill. It might be concluded from the relations shown by Plate I (in pocket) that the 
same formation extends continuously under the glacial and alluvial deposits past Breckenridge 
to Shock Hill. This, however, is not the case, for it is known that a large part of the older 
auriferous gravel between Breckenridge and French Creek rests on porphyry. On the other 
hand, dredging has shown that the quartzite of Nigger Hill is continuous with that exposed 
on the south slope of Prospect Hill. Another area extends over Gibson Hill and probably 
crosses under the gravels of the Blue to connect with the part of the formation that rests on 
the pre-Cambrian west of Braddocks. Other smaller fragments of the Dakota are exposed 
along the west side of the Blue from the Barton gulches to Shock Hill, along both sides of 
French Creek from its mouth to the vicinity of Lincoln, and at many places in contact with 
the great porphyry sheet of Bald Mountain. 



The section of the Dakota exposed near Breckenridge Pass, just west of Boreas, affords a 
total thickness of about 200 feet. The only locaUty within the district mapped where a fairly 
full section of the formation can be measured is in the railway cut through Rocky Point. Here 
the total thickness of the beds supposed to belong to the Dakota is about 230 feet. They are 
conformably overlain by black Upper Cretaceous shale, the contact being well exposed. The 
base of the visible section is an intrusive sheet of porphyry underneath which are '^ Wyoming" 
beds. 

It is possible that the Dakota sandstone as mapped in the Breckenridge district includes 
some beds belonging to the Morrison formation or to the Comanche series. 

UTHOLOOT. 

At Boreas the Dakota comprises the following units, numbered from the basal number up: 

Section of the Dakota near Boreat, 

Feet. 

3. Massive buff fine-giained sandstone 50 

2. Thin-bedded reddish and gray sandy limestone with some pebbly grit and a little dark 

shale. Weather pink or light red as a whole 100 

1. Light-gray or buff sandstone with pebbly streaks. Beds thinner than No. 3. Bottom 
bed, about 6 feet thick, contains abundant pebbles of white quartz mostly less than half 
an inch in diajneter. This bed varies from coarse sandstone to fine conglomerate 50 



200 

Farther north, within the boundaries of the mapped area, it has not proved possible to 
recognize the three divisions just given. This is perhaps due partly to the local dismember- 
ment of the formation by igneous intrusions so that portions of it only are present in any one 
mass and partly to the fact that few of the natural exposures, owing to the prevalent low 
angles of dip, show sections of the beds, many of them exhibiting little more than the loose 
detritus derived from some particularly hard quartzitic stratum. The chief obstacle, how- 
ever, in the way of any subdivision of the Dakota for the whole district is probably the muta- 

1 Bee Holmes, W. H., Ninth Ann. Rept. U. S. Oeol. and Geog. Surv. Terr., for 1875, 1877, p. 259; also Cross, Whitman, Stratigmphic results 
of a reconnaissance in western Colorado and Utah: Jour. Geology, vol. 15, 1907, pp. 635-637. 



86 GEOLOGY AND 0B£ DEPOSITS OF BBECKENBIDGE DISTRICT, COLORADO. 

bility of the formation. One variation noticeable in passing from Boreas into the mapped 
area is that the buff sandstone changes to hard white, gray, or buff quartzite. This quartzite 
is the distinctive feature of the Dakota throughout the district and, in consequence of its 
superior hardness, is as a rule much better exposed than the shaly and calcareous beds with 
which it is in most places closely associated. 

At Rocky Point the railway cut affords the following section of beds striking northwest 
and dipping 65"* NE. 

Section of the Dakota at Rocky Point. 

Feet. 
14. Thin-bedded gray quartzite; beds less than 1 foot thick; confonnably overlain by Upper 

Cretaceous shale 18 

13. Gray quartzite 3 

12. Thin alternating beds of gray quartzite and shale; beds generally less than 6 inches thick. . 25 

11. Quartzite 12 

10. Dark shale 1 

9. Quartzite 4 

8. Thin shaly sandstone and gray shale 1.5 

7. Massive quartzite 15 

6. Alternating thin gray quartzite and gray to black shale 18 

5. Massive gray quartzite, disturbed and broken near base 30 

4. Disturbed red and green shale. Possibly some faulting here 25 

3. Thin-bedded light-reddish sandstone and shale 40 

2. Buff cross-bedded sandstone with a few small quartz pebbles 20 

1. Brittle gray shaly limestone resting on porphyry sheet with igneous contact 20 

232. 5 

At several horizons, especially in the upper part of the section, the gray shales and thinner- 
bedded quartzite contain flakes of black carbonized vegetable material, but no identifiable 
plant remains were found. 

It is clear that this section is distinctly different from that at Boreas, given on page 35. 
Above the thick buff sandstone at Boreas the beds are very poorly exposed, but are known 
to include some gray limestone and a little gray sandstone. Possibly some of these beds Jiot 
included in the measured Boreas section are really equivalent to beds above No. 11 at Rocky 
Point. Even on that supposition there is no clear correspondence between the rest of the sec- 
tions. The uncertainty regarding the base of the Rocky Point section adds to the difficulties 
of comparison. At first the base of the Dakota was supposed to be the bottom of bed 5, all 
below that being reddish and all above gray. It was found impracticable, however, to carry out 
this distinction along the ridge southeast of Rocky Point, and the lower beds of the section, 
notwithstanding their reddish tint, were found to be so different from the intensely red and 
micaceous sediments below the porphyry sill that they were finally mapped with the Dakota, 
although they may in part belong to the Morrison formation. 

Overlying the *' Wyoming" along the west slope of Bald Mountain, and separating that forma- 
tion from the porphyry that forms most of the mountain, is a strip of the Dakota, shown in 
Plate I (in pocket). This strip is, as a rule, not well exposed, being covered in many places by 
porphyry detritus from the slopes above. The characteristic quartzite of the Dakota may be 
well seen, however, in the Gold Bell and Golden Edge workings east of the pre-Cambrian inlier 
of Illinois Gulch. In the Grold Bell the quartzite is interbedded with some gray shale and 
is overlain by limestone or calcareous shale, in places at least 30 feet tliick, above which is the 
base of the Bald Mountain porphyry sheet. In the Golden Edge mine inclines and drifts have 
followed the bedding of the quartzite under the porphyry. South of this mine, where the slope 
is steeper, the first rock to outcrop above the red ** Wyoming'' beds is a hard buff quartzite, 
apparently about 150 feet thick. Probably considerable shale that does not outcrop is associ- 
ated with this quartzite, which is overlain by nearly 100 feet of compact gray limestone inter- 
bedded with some calcareous gray shale and a little reddish shale. 

The Dakota of the area southwest of the Mountain Pride mine consists mainlv of nearly 
white quartzite interbedded with dark-gray shale containing coaly particles. A noteworthy 



PBE-CAMBBIAN AND SEDIMENTARY ROCKS. 37 

characteristic of the qiiartzite of this area as well as of most others to the north is a deep, close 
pitting of the upper and lower surfaces of the beds along certain bedding planes, due to the cor- 
rosion of the quartz by atmospheric water. The kind of surface produced by this solution is 
illustrated in Plate XXXI, A (p. 160). The action is not noticeable on freely exposed surfaces 
of the quartzite, but apparently takes place wherever percolating water charged with organic acids 
from its passage through the soil penetrates the rock along bedding planes and remains in con- 
tact with the quartz long enough to effect its solution. The etching is similar to that described 
some years ago by C. W. Hayes.* The white quartzite and gray shale of the Mountain Pride 
mine pass downward into a massive light-buff to pinkish sandstone, below which are the red beds 
of the "Wyoming. " All the beds are poorly exposed on this slope and the contact between the 
two formations is not visible. 

The Dakota as represented in Illinois Gulch, on Little Mountain, and on Nigger Hill is chiefly 
white or light-yellowish quartzite. With this is associated a good deal of gray shale, in part 
calcareous, which is much more noticeable in underground workings than at the surface. The 
proportion of interbedded shale is rather high in the vicinity of the Puzzle and Ouray mines, as 
appears in the mines, although little of this rock is visible on the surface. Little Mountain and 
the west knob of Nigger Hill are composed of nearly horizontal beds of white quartzite up to 8 
feet thick, interbedded with rather hard calcareous and siliceous gray shale. In the quartzite 
are a few thin conglomeratic bands with small quartz pebbles. These grade into the fine- 
grained quartzite. The lower slopes of both hills are cumbered with quartzite debris which 
conceals the rock in place. Material thrown out from some abandoned tunnels near the bottom 
of the slope consists in part of pink sandstone and shale resembling that near the base of the 
Rocky Point section* Similar material, largely shale, has been taken from a prospect shaft of 
imknown depth on Little Mountain. None of this material has the micaceous character of the 
typical "Wyoming" sediments. 

The upper part of Gibson Hill is chiefly white or gray quartzite, much of which weathers 
rusty from the oxidation of disseminated pyrite. On the summit of the hill is a bed of dark 
grit in the quartzite, composed of angular bits of dark chert, up to a quarter of an inch in diameter, 
embedded in a matrix of smaller quartz grains. On the southwest slope of the hill this quartzite 
is associated in obscure stratigraphic relation with conglomeratic varieties containing scattered 
pebbles of white quartz and with shales that are in part micaceous. The dumps of the aban-. 
doned tunnels on the west slope of the hill show some reddish sandstone and shale, whose strati- 
graphic relations to the typical Dakota quartzite are unknown. Some shafts appear to have 
gone through this red material into quartzite below it. The crest of the broad spur extending 
from the north slope of Gibson Hill toward Braddocks is mostly hard white quartzite associated 
with some shale and containing at least one bed, fully 4 feet thick, of very hard conglomerate 
made up of quartz pebbles about the size of peas. 

The quartzite areas west of the Blue are, as a rule, so poorly exposed that little can be learned 
of their lithology or stratigraphy. On the northeast side of Shock Hill, according to Mr. Bastin, 
there is a little purplish-red sandy shale interbedded with the quartzite. He notes also that at 
the base of the quartzite, resting on the pre-Cambrian, is a layer of gray grit or fine conglomerate 
associated with gray Umestone and brick-red shale. The greatest thickness of conglomerate 
actually observed was 2 feet. The imperfectly rounded pebbles are chiefly quartz and pink 
feldspar, are generally less than an inch in diameter, and appear to have been derived from the 
pre-Cambrian rocks near at hand. There is perhaps some question whether these basal beds 
west of the Blue are part of the Dakota or represent the thin edge of the "Wyoming.'' It was 
not found practicable, however, to separate them from the Dakota quartzite in mapping. 

In conclusion, the Dakota in the Breckenridge district is generally a hard, fine-grained, white 
or gray quartzite. At many places this is the only rock visible; in others it is interbedded with 
gray or more rarely red shale, and may contain calcareous or conglomeratic beds. 

1 Solution of silica under atmospheric conditions: Buil. Qeol. Soc. America, vol. 8, 1897, pp. 213-220. 



38 GEOLOGY AND ORE DEPOSITS OF BRECKENKIDGE DISTRICT, COLORADO. 

UPPER CRETACEOUS SHALE. f 

PBEUMIFA&T STATEMENT. 

In the Breckenridge district the beds overlying the Dakota are dark shales with a few very 
subordinate thin layers of Umestone and of gray quartzite. A few fossils known to be distinctly 
Benton, others fairly well determined as Niobrara, and some imperfect specimens that are 
possibly Montana species have been found in these beds in the course of the present investigation; 
but neither the available paleontologic evidence nor the lithologic character of the series justifies 
a local attempt at subdivision. The section is not only greatly affected by igneous intrusions, 
but it has no definite stratigraphic top, the beds, which dip generally to the east, being faulted 
down on that side against the pre-Cambrian. In view of these facts and of the great thickness 
of the series, which, as will presently be seen, much exceeds any that has been recognized for the 
Colorado Group alone in this latitude, the names given to the divisions of the Cretaceous above 
the Dakota in the central Great Plains region are not here appUcable. The development of the 
shale strongly suggests the Mancos of western Colorado, but, for reasons given in the section on 
correlation, the use of the name Mancos shale for the formation near Breckenridge is scarcely 
warranted by present knowledge of the stratigraphy of central Colorado. Accordingly, it has 
seemed best to refer to the shale in this report as Upper Cretaceous shale, or occasionallj" more 
briefly in the body of the text as black shale. If subdivision into smaller units is ever effected 
this will probably be by approach from the north, where along the lower course of the Blue, 
as the work of Marvine indicates, there are sections more suitable than those near Breckenridge 
for stratigraphic study. 

DZSTBIBX7TI0N. 

The Upper Cretaceous shale, very intricately intruded by porphyry, occupies the northeast 
half of the district with the exception of a small area of pre-Cambrian, against which the shale is 
faulted, in the extreme northeast comer. (See PI. I, in pocket.) 

Not only has the porphyry invaded the shale in an exceedingly irregular maimer, but the 
igneous rock has inclosed innumerable small masses of the sedimentary rock, of which a few 
only of the larger and better exposed ones can be shown on the geologic map. Along the eastern 
border of the district the shale extends continuously from the head of French Gulch over Fam- 
comb Hill and across the Swan to Muggins Gulch. 

Some Upper Cretaceous shale occurs also in the southwest half of the district on Shock Hill, 
between Prospect and Gibson hills, in Illinois Gulch, and in a belt extending from Little 
Mountain past Rocky Point to Bacon. It is probably the principal bedrock of French Gulch 
between the Wellington or Country Boy mine and Lincoln, but here, as in many other places, its 
exposures are very inconspicuous and the shale is more subject than other rocks in the district 
to concealment by soil and forest. 

LITHOLOOT. 

The Upper Cretaceous shale is generally dark gray to black, carbonaceous, calcareous, 
highly fissile, and rather soft, breaking into small flakes when exposed for a few years to the 
weather. Much of it as seen in fresh underground exposures is coal-black. It exhibits, how- 
ever, considerable variation. This is due in part to metamorpliism by the intruded porph^^ry 
masses, but in part also to original differences in composition. Some is hard, light gray, and 
siliceous, grading into fine-grained gray quartzite or into compact chert\^ and calcareous beds. 
Such varieties lack the fissility of the softer shale and, as they resist erosion better, they may 
form steep slopes and outcrop on peaks and along ridges, as on Brewery Hill or on the spur of 
Mount Guyot between French and Little French gulches. Thin lenses of fetid fossiliferous 
limestone occur at many localities and do not appear to be specially characteristic of any 
particular horizon in the formation. The color of the weathered shale is far from uniform. 
Although generally black or somber gray, the formation in some places weathers red-brown, 
buff, or light gray. With this preliminary general characterization of the shale attention may 
now be given to a few of its local variations. 



PRE-CAMBRIAN AND SEDIMENTABY ROCKS. 39 

The shale of the Rocky Point, Gibson Gulch, Gold Run, and Delaware Flats areas is nearly 
all of the soft, black fissile variety that in the development of the topography tends to the 
formation of ravines and swales with no projecting outcrops. An exception to this is found 
on the hill east of Gibson Gulch, where the shale, apparently metamorphosed by intrusions of 
diorite porphyry, is hard, cherty, and of light greenish gray color. Similar soft black shale 
occurs along French Gulch and in the vicinity of Lincoln Park. Much of the shale of Famcomb 
Hill also is of similar kind but on the whole is a slightly harder rock that grades to the north, 
east, and south into some of the most resistant f acies of the formation. The lower west slope of 
Mount Guyot, Little French Gulch, and the steep spur east of Monitor Gulch are all carved 
from hard, ringing dark shales very different in appearance from those exposed along the 
bottoms of American and Georgia gulches. Yet the one set of beds strikes directly into the 
other and the clinking shales of Mount Guyot, which weather in warm red-brown tints, are 
stratigraphically continuous with the gray banded cherty shales east of Monitor Gulch and with 
the black shales of American and Georgia gulches, which split into fragile flakes where laid 
bare by placer mining. These in turn strike into the generally hard and rather flinty shales of 
Brewery Hill. 

On the south side of the Swan, just east of Rock Island Gulch, the shales are well exposed 
in the sides and bottom of a large ditch or small canal. Here they are rather sandy, yellowish 
brown in color, and flecked with shadowy dark spots that possibly are fragmentary plant remains. 
This same band of brown sandy shale can be traced southward past Snyder's placer camp, near 
bench mark 9823. 

North of the Swan, in the vicinity of Muggins Gulch, the shales are rather variable. They 
contain, together with a good deal of fissile black shale, some bands of fine-grained dark-gray 
quartzite, apparently not of great persistency, some hard dark shale identical with that on the 
west slope of Mount Guyot, some very compact gray bands suggestive of silicified calcareous 
shale, and a little gray limestone. The assemblage on the ridge east of Muggins Gulch sug- 
gests on the whole the hard beds of upper French Gulch or of the northeast slope of Brewery 
Hill rather than the fissile shales of Famcomb Hill; but, as is commonly the case in the shale 
areas, soil and vegetation greatly obscure the details in the relations of the various beds. 



It is not possible to make an accurate estimate of the thickness of the Upper Cretaceous 
shale in the Breckenridge district. Even were the total thickness measurable the figure 
obtained would represent only a part of the whole, as some of the upper beds of the formation 
are known to have been removed by erosion. The distance across the strike of the shale from 
the diorite porphyry south of Famcomb HiU to the end of the spur east of Monitor Gulch is 
about 8,000 feet. The observed dips along this section range from 35° to 60° and the average 
may be taken as 45°. The dip is uniformly northeast and as tliere are no large intrusions of 
porphyry crossed by the section the horizontal distance of 8,000 feet corresponds to a thickness 
of about 5,500 feet, provided that there is no important duplication of beds by faulting. Similar 
measurements at the head of French Gulch and over Brewery Hill give results of the same 
order of magnitude. In the steep slope east of French Gulch and south of Little French Gulch 
the beds are well exposed, dip uniformly to the east, and. present no evidence of important 
faulting; yet this section alone indicates a tluckness of 3,500 feet. If measurement is confined 
merely to the spur east of Monitor Gulch, where it appears impossible that any duplication by 
faulting could escape notice and where the dip is very regular, a thickness of 3,000 feet is 
obtained. In comparison with these figures, the maximum thickness of the Colorado group 
in eastern Colorado as given by Darton * is 1,430 feet, or less than half the thickness calculated 
for the Breckenridge district. Tins discrepancy is so great as to render it necessary to suppose 
either that faulting, resulting in repetition of the beds, has been much more important than 
study of the surface relations of the rocks indicates, or that the dark shales of the district 
include beds corresponding to part of the Montana group, and thus represent a development 

1 Darton, N. H., Oeol(^y and underground waters of the central Great Plains: Prof. Paper U. 8. Geol. Survey No. 32, 1905, p. 76. 



40 GEOLOGY AND ORE DEPOSITS OF BRECKENRmGE DISTRICT, COLORADO. 

similar to that of the Mancos shale in western Colorado. The latter is regarded as the more 
likely supposition of the two. Probably when detailed geologic work is taken up 8 or 10 miles 
north of Breckenridge, where the shale is extensively exposed and apparently is less disturbed 
and indurated by porphyry intrusions than in the area now under investigation, a much more 
satisfactory estimate of the thickness and constitution of the formation can be made. 

COBBELATION. 

Along the eastern front of the Rocky Mountains, in the central Great Plains region, that 
part of the Upper Cretaceous lying above the Dakota and below the '^Laramie" is generally 
divisible into two groups — the Colorado group, in part subdivisible into the Benton and Nio- 
brara formations, and the Montana group, subdivisible into the Pierre and Fox Hills formations, 
or, in some parts of the field, into more numerous units. At Morrison, 45 to 50 miles east of 
Breckenridge and on the opposite side of the Front Range, Eldridge * found above the Dakota 
(a) 500 feet of dark Benton shale, (6) 400 feet of Niobrara, consisting of limestone, gray marly 
clays, and yellow or buff shales, (c) 7,700 to 7,900 feet of Pierre, consisting of plastic lead-gray, 
bluish and yellowish clays with a zone of sandstone from 100 to 350 feet thick about one-third 
of the way from the base to the top of the formation, and (d) 800 to 1,000 feet of the Fox 
Hills formation, chiefly arenaceous shale. 

In the vicinity of Breckenridge the geologists of the Hayden Survey evidently saw no 
sedimentary rocks above the Dakota and mapped the shale and intruded porphyry of the 
northeastern part of the district as Archean. South and north of the Breckenridge area, 
however, the Hayden atlas shows the Dakota as overlain by the Colorado group and this by 
the Laramie, no Montana (Fox Hills or Pierre) being indicated. 

Eldridge ' in 1894, as a result of his study of the Anthracite-Crested Butte area, 50 miles 
southwest of Breckenridge, on the other side of the Sawatch uplift, described the Dakota as 
overlain by 150 to 300 feet of dark Benton shale. This is succeeded, according to Eldridge, 
by the Niobrara formation, consisting of 20 to 40 feet of limestone overlain by 80 to 160 feet 
of gray calcareous shale. Above these is the Montana group, embracing about 2,500 feet of 
leaden-gray clays with numerous lenses of limestone assigned to the Pierre and about 300 feet 
of alternating clays and sandstones assigned to the Fox Hills. 

In the Aspen district, just north of the Anthracite-Crested Butte area, Spurr' made a 
similar division of the Cretaceous rocks above the Dakota. Here he found black calcareous 
Benton shale 350 feet thick, succeeded by dense limestone 50 to 75 feet thick, overlain by shaly 
limestone. This limestone, with a total thickness of about 100 feet, Spurr assigned to the 
Niobrara. Above the Niobrara is a great thickness, probably near 4,000 feet, of gray or black 
shale with thin lenticular beds of limestone. This is referred to the Montana group, the Pierre 
and Fox Hills formations not being distinguished. 

It thus appears that the Breckenridge district lies between the Great Plains region on 
the east and the Crested Butte and Aspen region on the southwest, in both of which the Upper 
Cretaceous beds are susceptible of the same general division into the Dakota, the Colorado 
group, containing the Benton and Niobrara, and the Montana group. It would naturally 
be expected that the beds at Breckenridge would be separable into the same units. This is 
not the case; but before the divergency is discussed attention may be turned for a moment 
to western Colorado. 

When Cross * in 1899 published the first of his series of folios on the San Juan region, in 
southwestern Colorado, he described nearly 2,000 feet of dark shale above the Dakota, to 
which he gave the name Mancos shale. This was shown to carry fossils elsewhere characteristic 
of the Benton, Niobrara, and Pierre formations but to be not divisible into equivalent litho- 

^ Emmons, S. F., Cross, Whitman, and Eldridge, 0. H., Geology of the Denver Basin in Colorado: Mon. U. S. Geol. Survey, vol. 27, ISdd, 
pp. 51-72. 

> Anthracite-Crested Butte folio (No. 9), Oeol. Atlas U. S., U. S. Geol. Survey, 1894. 

• Spurr, J. £., Geology of the Aspen mining district, Colorado: Mon. U. S. Geol. Survey, vol. 31, 1898, pp. 41H13. 

« Cross, Whitman, Telluride folio (No. 57), Geol. Atlas U. S., U. S. Geol. Survey, 1899. See also folios 60, ISO, and 153. 



PBB-CAMBRIAN AND SEDIMENTABY ROCKS. 41 

logic units. When Fenneman and Gale* took up the study of the Yampa coal field in 
northwestern Colorado they were confronted with stratigraphic conditions similar to those in 
southwestern Colorado. In other words, they found overlying the Dakota approximately 2,500 
feet of shale, with very subordinate limestone and sandstone members, which they correlated 
with the Mancos shale. Taff * has described similar relations in central Utah, the coal-bearing 
Mesaverde formation being underlain by fully 1,600 feet of Mancos shale, and Gale ' has since 
extended his earlier observations on the Mancos in the extreme western part of Colorado, 
where he finds the shale to attain a thickness of about 5,000 feet. The general result of all 
these studies has been to show a decided difference in the lithology of the Colorado and Mon- 
tana groups of the Cretaceous in eastern and western Colorado. One of the first stratigraphic 
questions that arose when field work at Breckenridge was begun was whether the post-Dakota 
Cretaceous would be found to correspond to the Great Plains section, such as is exposed only 
45 miles away on the other side of the Front Range, or whether it would correspond to the 
sections of western Colorado, with the Mancos shale, representing all of the Colorado and part 
of the Montana group. The latter appears to be the case, there being no distinct and per- 
sistent calcareous formation recognizable as the Niobrara, and were it not for the work of 
Eldridge at Crested Butte and of Spurr at Aspen the shale at Breckenridge would be referred 
to the Mancos. Under the circumstances outlined, however, it is deemed best to defer actual 
correlation until more is known of the Cretaceous section north and south of Breckenridge, 
particularly as in that district the conditions for stratigraphic study are far from favorable 
and the original lithologic character of the shale may have been considerably modified by the 
porphyry intrusions. 

Fossils are not abundant in the black shale as developed near Breckenridge, but have been 
found at a few places in different parts of the district. The small collections made were sub- 
mitted to Dr. T. W. Stanton, who kindly identified the species represented. 

In a small cut close to the railway and 1,500 feet southeast of the summit of Little Mountain, 
thin-bedded limestone and calcareous shale, interbedded with softer black shale, carry imper- 
fectly preserved shells of Inocerajnus deformis Meek (?). If correctly identified, this fossil is 
indicative of the lower part of the Niobrara, according to Mr. Stanton. The beds strike N. 55® 
W. and dip 60° SW. They are intruded by porphyry and their stratigraphic height above the 
Dakota is not determinable from their field relations, although these are more suggestive of 
the lower than of the upper part of the formation. 

In thin dark fetid limestone 500 feet north of the Rocky Point cut and below the railway 
were collected Ostrea hiffuhris Conrad, Inoceramus fragUis H. and M., and ScapJiites warreni 
M. and H. These, according to Mr. Stanton, are upper Benton species. Here again proximity 
to intrusive porphyry renders useless an attempt to fix by ordinary stratigraphic observations 
the exact place of the fossils in the geologic column, but there is nothing in the visible structure 
to suggest that they occur at a horizon any lower than that of the Inoceramus deformis near 
Little Mountain. A bed of fetid limestone identical in appearance with that near Rocky Point 
occurs with dark shale in the railway cut 500 feet south of Bacon. Although it contains abun- 
dant remains of shell fragments no identifiable fossils could be found. 

In the dark shale on the south edge of Delaware Flats Mr. Bastin collected specimens of 
Inoceramus labiaiu,s Schlotlicim (?), and the same fossil was found in harder shale 1,000 feet 
east of the head of Monitor Gulch, east of Famcomb Hill. This species, if correctly identified, 
indicates beds lower in the Benton than those carrying Ostrea lugubris, according to Mr. Stanton, 
whereas stratigraphically it appears in this district to be liigher and occurs about 4,000 feet 
above the Dakota, provided no faulting has essentially modified the structure. Mr. Stanton 
states, however, that the determination is uncertain and that the specimens doubtfully referred 
to Inoceramus labiatus may really be a Montana form. 

1 Fenneman, N. M., and Gale, U. S., The Yampa coal field, Routt County, Colorado: Bull. U. S. Qeol. Survey No. 297, 1906. 
> Tall, J. A., The Pleasant Valley coal district, Carbon and Emery counties, Utah: Bull. U. S. Oeol. Survey No. 316, 1907, p. 341. 
< Oale, H. S., Geology of the Rangely oil district, Rio Blanco County, Colorado, with a section on the water supply: Bull. U. S. Qeol. Survey 
No.350, 1908, pp. 26-32 



42 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

Inoceramus deformis Meek has also been found poorly preserved in shale 200 feet north 
of Lincoln, and in better, though distorted, specimens, definitely identified by Mr. Stanton, 
in Summit Gulch near its mouth. 

A single specimen of InocerarmLS collected by Mr. D. Foster Hewett near the summit of 
Farncomb Hill, just west of the head of Monitor Gulch, is not well enough preserved for iden- 
tification, but according to Mr. Stanton suggests the Montana group species, such as Inoceramus 
cripsii, rather than any common species of the Colorado group. Fragments, apparently of the 
same fossil species, were noted at one or two places in the loose shale on the south slope of Farn- 
comb Hill, east of the Wire Patch mine. 

ThiB fossils found near Breckenridge thus indicate that the shale represents both Benton 
and Niobrara time and possibly also part of Montana time, although neither the paleontologic 
nor the stratigraphic data suffice for local subdivision into the corresponding formations. 
Paleontology and lithology both suggest the same conclusion — thai in the region drained by 
the Blue the Cretaceous beds above the Dakota show the development characteristic of western 
and not of eastern Colorado. If true, this extends the known range of the Mancos phase of sedi- 
mentation to the east and brings it within 45 miles of the eastern phase as exemplified by the 
Morrison section. The relation of the Breckenridge section to that at Crested Butte is a problem 
awaiting investigation. 



CHAPTER III. 
PETROGRAPHY OF THE IGNEOUS ROCKS. 

GENERAL FEATURES. 

EARLIER STUDIES. 

The porphyritic intrusive rocks of the Breckenridge district have certain characteristics of 
composition and texture that, as Cross ^ has shown, are common to the late Cretaceous or Ter- 
tiary intrusives of a region stretching from Boulder County southwestward across the State of 
Colorado into Utah and Arizona. Recently S. H. Ball ^ has reviewed the literature of these 
porphyries and has approximately outlined the belt of their occurrence in central Colorado. 
In the same report J. E. Spurr and G. H. Garrey emphasize the geographic coincidence of 
this belt of generally monzonitic porphyries with the principal metalliferous deposits of the State 
and show that there is probably a genetic relation between the two. 

In the Tenmile district the porphyries, according to Cross,' grade from the more saUc to 
the more femic varieties. Three types, the Lincoln, Elk Mountain, and Quail porphyries, are 
in that area distinguished and mapped by Emmons,* but it is clear from his descriptions that 
these are variants from one general monzonitic magma and are connected by intermediate 
facies. In the Leadville district Cross ^ distinguished (1) quartz porphyry or Mount Zion 
porpyhry, (2) white or Leadville porphyry, (3) pyritiferous porphyry, (4) Mosquito porphyry, 
(5) Lincoln porphyry, and (6) gray porphyry. This classification depends in part on local 
alteration of the porphyries in the vicinity of the Leadville ore bodies and is of little use out- 
side of that district ; the only one of the six types that appears to be generally recognizable north 
of the Leadville area is the Lincoln porphyry, with its characteristic large phenocrysts of ortho- 
clase and smaller though conspicuous grains or crystals of quartz. Neither at ^Leadville nor in 
the Tenmile district do the geologists appear to have discovered evidence that any of the types 
of porphyry are distinctly older or younger than others. 

RELATION TO PORPHYRIES OF NEAR-BY DISTRICTS. 

The intrusive rocks of Breckenridge are not only in the same general belt or province as 
those of Leadville and the Tenmile district, but they belong with them to one local petrographic 
field, and the geologist passing southward from Breckenridge to Leadville by way of Mounts 
Silverheels and Lincoln finds the same familiar kinds of porphyry recurrent along his route. 
This holds true also between Leadville and the Tenmile district. Between the Tenmile Valley 
and that of the Blue at Breckenridge intervenes the pre-Cambrian ridge of the Tenmile Range, 
which constitutes a local barrier between the intrusives in the sediments of the tw^o districts. 
Probably, however, a close examination of this range would show some direct connection 
between the porphyries of the Tenmile and Breckenridge districts through intrusions in the 
pre-Cambrian rocks. 

^ Cross, Wliitman, The laocolitic mountam groaps of Colorado, Utah, and Arizona: Fourteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1894, pp. 
165-241. 

* Prof. Paper U. S. G«oI. Survey No. 63, 1908, pp. 67-70, PI. XI. 
« Op. cit., pp. 222-224. 

* Tenmile special district folio (No. 48), Geol. Atlas U. S., U. 8. Geol. Survey, 1898. 
6Mon. U. 8. Geol. Survey, vol. 12, 1886, pp. 32^-333. 

43 



44 GEOLOGY AXD OBE DEPOSITS OF BBECKENBIDGE DIBTBICT, COLOBABO. 

TTraSS OB VARIETIES. 

Within the Breckenridge district the intrusive porphyries may conveniently be considered 
as belonging to three types. These are (1) the silicic type, exemplified by quartz monzonite 
porphyry with phenocrystic quartz; (2) the calcic type, exemplified by monzonite porphyry, 
locally grading into diorite porphyry, with only incidental quartz in the groundmass; and (3) 
the intermediate type in which quartz forms very inconspicuous phenocrysts or belongs solely 
to the groundmass. Although, as will be seen, these varieties are not all of precisely the same 
age they are connected by many intermediate facies and are believed to be closely related as 
slightly differentiated products of one magma. 

QUARTZ MONZONITE PORPHYRY. 

HEGASCOPtC CHABACTEB. 

The sihcic type of porphyry (Lincoln porphyry of the Tenmile folio) is generally a light- 
colored rock, being as a rule pale gray and weathering to buff or brown tints or to a fainter 
sliade of green than the other porphyries. (See PI. VI. ) It is as a rule conspicuously porphyritic. 

Rounded or bipyramidal phenocrysts of 
quartz up to a centimeter in diameter, 
lar^e well-formed crystals of orthoclase up 
to 8 centimeters in length, and smaller 
phenocrysts of plagioclase contrast con- 
spicuously with the generally rather dense 
fine-grained groundmass. In some vari- 
eties small phenocrysts of biotite, the scales 
being rarely over 3 millimeters across, are 
abundant; in others dark plienocrysts are 
nearly absent. Hornblende is visible in 
some facies but is not characteristic of the 
silicic type and where present appears to 
indicate a transition toward the calcic 
type. The principal field criterion for the 
distinction of the quartz monzonite por- 
phyry from the monzonite porphyry is the 
presence of phenocrysts of quartz and 
orthoclase, this feature commonly being 
associated with characteristic differences 
in color and texture. As a rule there is no 
difficulty in distinguishing these two kinds 
of porphyry, the rock of the large irregu- 
lar intrusive mass exposed along both sides 
of the Swan, for example, being decidedly different in color, texture, and mode of weathering 
from that forming the upper part of Bald Mountain and the slopes of French Gulch west of 
Lincoln. Other masses, however, as will be shown later, are intermediate in character between 
the two types. 

HICBOSCOFICAI. CHABACTEB. 

Under the microscope the silicic type of porphyry generally shows a sharper distinction 
between phenocrysts and groundmass than does the calcic type. The phenocrysts are com- 
paratively large and well formeii, and the groundmass is fine graiJied and equally granular. 
Consisting almost wholly of quartz and orthoclase, the groundmass presents the appearance 
(see fig. 2) of a fine mosaic of closely fitting irregular grahis, of which the average diameter 
ranges in various specimens from 0.02 to 0.08 millimeter. The absence of the more or less 
euhedral lath-shaped sections of plagioclase found in the calcic type results in a noticeably 




leol quaiu 
□ Motion, with nlpolj crossed, 
q, QuarCi: p, pbglodsse; b, 



porphyry <Jlllclc type) 



3. S. QEOLOaiCAL SURVEV 



PROFESSIONAL P 




A. BROWNS GULCH NEAR CASHIER MINE. 

Shows conapicuoui phenocrysta of oithoclasa. Natural size, Saa page 49. 




B. EAST SLOPE OF BREWERY HILL, 
Mote granular teilure than specimen shown above. Natural size. Described with che 
analyses on pages 45-49. 

QUARTZ MONZONITE PORPHYRY, SILICIC VARIETIES. 



PETROG3APHY OF THE IGNEOUS ROCKS. 



45 



diflFerent texture of the groundxnass, as may be seen by comparing figures 2 and 3 (pp. 44, 61). 
Neither poikilitic nor xnicropegmatitic textures are characteristic of the groundmass in the 
silicic type. 

The very conspicuous phenocrysts of orthoclase in this rock, owing to their size, do not 
as a rule appear in thin sections, where phenocrysts are aknost exclusively plagioclase, gener- 
ally andesine, corresponding to the formula Ab^Anj. In other words, orthoclase occurs as , 
large megascopic phenocrysts and in the groundmass, but not as a rule in phenocrysts such as 
are ordinarily included in microscopical sections. Phenocrysts of quartz, generally showing 
more or less magmatic corrosion, are abundant. Nearly all thin sections show some biotite 
phenocrysts, but hornblende is exceptional. In the silicic type allanite is more common and 
occurs in larger crystals than in the calcic type. 

CHEMICAL COMPOSITION AND CLASSIFICATION. 

The chemical composition of the silicic type is represented by the following two analyses: 

Chemical analyses of quoarU momonite porphyry. 
[R. C. Wells, analyst.) 



SiOi 67. 53 

AliOa 15.46 

FeiOj 2.18 

FeO 2. 42 

MgO o . 16 

CaO 3.24 

NajO 3.24 

KjO 3.86 

H«0- .23 

H,0+ 55 

TiOi . .41 

ZrOi -02 




99.63 



0.03 


0.22 


.01 


.17 


.09 


1.52 


.03 


None. 


None. 




None. 


.01 


.10 


.12 


.07 


.03 


None. 


.03 



99.65 



a Probably a little low.— F. L. R. 

1. Qaartx monxonite porphyry. Amlatose (Br. 104). Brewery Hill, 1,000 feet northeast of summit. 

2. Quartx monxonite porphyry. Toscanose (Br. 110). Browns Oulch, 900 feet south of Swan City. 

Sample 1 was taken from some large fresh masses blasted from the roadway. This rock, 
shown in Plate VI, B (p. 44), is bright gray, speckled with small crystals of biotite, and has 
almost a granitic texture and appearance. Rather unevenly distributed through it are large 
crystals of orthoclase up to several inches in length and generally twinned in accordance with 
the Carlsbad law. These are more abimdant on the simamit of the knob northeast of Brewery 
Hill than at the exact spot where the sample was taken. In addition to the large crystals are 
abundant phenocrysts of quartz and plagioclase up to a centimeter across, many smaller ones 
of biotite, and rarely a small prism of hornblende. The rock is much nearer monzonite or 
granite in textiu'e than most of the quartz monzonite porphyry of the district. 

Under the microscope in thin section (see PI. VII, B) the most abundant large pheno- 
crysts are plagioclase, the Orthoclase phenocrysts, owing to their size, being not as a rule included 
in the chips used for microscopic sections. The plagioclase in the specimen analyzed shows some 
zonal structure and has not been sharply determined. Its refractive index and extinction 
angles, however, indicate an andesine or sodic labradorite. The crystals are subhedral to 
anhedral, and are twinned in accordance with the albite, Carlsbad, and pericline laws. Some 
of the plagioclase phenocrysts are really aggregates of anhedral crystals, which as a rule poiki- 
litically inclose grains of quartz and crystals of biotite. The quartz is wholly anhedral and is 
confined to the groundmass, in which the average diameter of grain is about 0.3 millimeter. 
The orthoclase, exclusive of the large phenocrysts, is also anhedral, with a tendency to form 
poikilitic aggregates wdth the other minerals. In contrast with the clear plagioclase it has gen- 
erally a gray turbid appearance. It is not noticeably pertliitic. The hornblende and biotite, 
which are in part intergrown, present no features of special note. Other minerals microscop- 
ically visible are magnetite, pyrite, apatite, titanite, zircon, allanite, and a very small quantity 
of calcite and chlorite. 



PLATE VII. 

Photomicrographs of Porphyries. 

A. Monzonite porphyry, near diorite porphyry. Wellington mine. X40. Nicola croesed. 

lUuBtration shows little more than the general texture of the rock, which contains hypersthene. For description 
see page 55. 

B. Quartz monzonite porphyry. East slope of Brewery Hill. X40. Nicols crossed. 

Described with chemical analysis on page 45. o, Orthoclase; p, plagioclase; q, quartz; hi, biotite. Illustrates 
coarsely crystalline siliceous variety. / 

C. Quartz monzonite porphyry. Slope east of Iloosier Pass. X40. Nicols crossed. 

Illustrates well the usual texture of the siliceous variety, p, Plagioclase; bi, biotite; q, quartz; py, pyrite; ap, 
apatite. The biotite is partly chloritized. 

46 



J. S. QEOLOOICAL 




•^^ 


w 


Hu^^S^^BKfj'y 


« 

< 


P'5i..^>3 




PHOTOMICROGRAPHS OF PORPHYRIES. 



PETROGRAPHY OF THE IGNEOUS ROCKS. 47 

The ideal norm of the quartz monzonite porphyry of Brewery Hill, calculated from 
analysis No. 1 for the purpose of ascertaining the place of the rock in the American system * 
of classification, is as follows: 

Norm of quartz monzonite porphyry of Brewery Hill. 

Quartz 25. 56 

Orthoclafie 22. 80 

Albite 27. 25 

Anorthite 16.12 

Hypersthene 2. 51 

Magnetite - 3.25 

llmenite 76 

98.25 

Pyrite * .09 

Water 78 

99.12 

In the terminology of that system the rock is persalic, quardofelic, alkalicalcic (but near 
domalkalic), and sodipotassic. It is thus amiatose but is near toscanose. 

The rock is wholly crystalline, and it is possible, with the aid of the microscopical study, 
to calculate approximately from the chemical analysis the actual mineralogical composition. 
For this purpose it is assiuned that the hornblende in the rock is of the same composition as 
the hornblende in the quartz monzonite near Mount Hoflfmann, CaL, and that the biotiteis 
identical with the biotite in the quartz monzonite of the Nevada Falls trail, Yosemite Valley. 
These are as follows: 

Analyses of hornblende and biotite. 



SlOj... 

AlsOfc, 

Fe,0,.. 

FeO... 

MgO.. 

CaO... 

NasC. 

KjO.. 

Ha0+ 

HaO-. 

Ti0»... 

P,0a.. 



47.49 

7.07 

4.88 

10.69 

13.06 

11.92 

.75 

.49 

1.86 



1.21 



35.75 

14.70 

4.65 

14.06 

12.37 

.17 

.32 

9.19 

3.64 

1.03 

3.16 

.03 



Cr,0,. 

NiO.. 

MnO. 

BaO. 

F.... 

V,0,. 



O-F. 



0.02 
.51 



.06 
.04 



100.05 
.02 



100.03 



Trace. 
0.02 
.45 
.12 
.17 
.06 



99.90 
.07 



99.83 



1. fiombkode. From quartz monzonite (toacanoee), Tioga Road ^ southeast of Mount Hoffmann, Cal. W.. F. HiUebzaad, analyst. Turner, 
H. W., Am. Jour. Sci., 4th ser., vol. 1, 1899. p. 297; Cross, Iddings, Pirsson, and Washington, op. cit.. Table XIII, a,* Bull. U. S. Oeol. Survey No. 
168, 1900, p. 206; Bull. U. S. Geol. Survey No. 419, 1910, p. 207. 

2. Biotite. From quartz monzonite (toscanose), Nevada Falls trail, Y^mite Valley. W. F. Hillebrand, analyst. Turner, H. W., Am. • 
Jour. Sci., 4th ser., vol. 7, 1899, p. 294; Cross, Iddings, Pirsson, and Washington, op. cit., Table XIV, 6.- Bull. U. S. Geol. Survey No. 419, 1910, p. 288. 

The preliminary calculation gives the following: 



Tentative mineralogical composition of quartz mjonzonite porphyry from Brewery Hill. 



Per cent 
by weight. 

27.14 



Quartz 

Orthoclase 20.57 

llbite 27.25 

Anorthite 14. 46 

Biotite 3. 46 

Hornblende 1. 11 

Magnetite 3. 02 

Titanite... 98 

Pyrite 09 

HgO, not used in calculating minerals 74 



Apatite, zircon, allanite, etc 



98.82 
.81 

99.63 



» Cross, Whitman, Iddings, J. P., Pirsson, L. V., and Washington, II. S., The quantitative classification of igneous rocks, Chicago, 1903. 



48 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

The calculation requires 0.48 per cent more magnesia and 0.79 per cent less ferrous oxide 
than are reported in the chemical analyses, so that the mica actually present is perhaps more 
ferruginous and less magnesic than the biotite from the California rock. 

In this statement of composition the feldspars are given as if they were chemically pure. 
In reality most of the albite is combined with the anorthite as andesine or labradorite, and there 
is the possibility of a small remainder being combined with the potassium-aluminum silicate 
as orthoclase. Hillebrand^ has analyzed the orthoclase phenocrysts from the *'gray porphyry" 
of Johnsons Gulch, near Leadville, as follows: 

Chemical analysis of orthoclase. 

SiOa 66. 22 

AI2O3 20. 33 

CaO.. 2 95 

NaaO 3. 45 

K2O 8. 31 

H2O 1.90 

99.16 

The lime in this analysis shows that some plagioclase was present in the material used. 
The molecular ratio of CaO to NajO is 52 to 55, corresponding nearly to the plagioclase formula 
AbjAni, or andesine. In other words, the soda present is not in excess of what might reasonably 
be supposed to be combined with lime in the plagioclase molecule, and the orthoclase itself 
presumably contains little soda. This agrees with the general absence of perthitic intergrowths 
shown by microscopical studies of these porphyries and with the observed inclusion of plagio- 
clase in some of the large orthoclases. Inasmuch, however, as chemical analyses of rock-making 
orthoclase very rarely show less than 1 per cent of NajO, it is safe to include at least this quantity 
in the Breckenridge mineral and consequently to assume that for every 10 molecules of potash 
in the orthoclase there is present one molecule of soda.' On this supposition the mineralogical 
composition of the porphyry becomes as follows: 

Mineralogical composition of quartz mxmzonite porphyry from Brewery Hill. 

Quartz 27. 14 

Orthoclase 22. 67 

Andesine (Abfl8An34, or nearly AbaAnj) .' 39. 61 

Biotite 3. 46 

Hornblende 1. 11 

Magnetite 3.02 

Titanite .98 

Pyrite S 09 

H2O, not used in calculation * 74 

98.82 
Apatite, zircon, allanite, etc 81 

99. 63 

» Cross, Whitman, Mon. U. 8. Geol. Survey, vol. 12, 1886, p. 333; Bull. U. S. Geol. Survey No. 419, 1910, p. 257. 

* That b, the composition of the orthoclase is supposed to b^— 

Per Molecular 
cent. r.itios. 

SlOf : 64.40 1.068 

AltOi 18. 87 .185 

KjO 15. 73 .067 

NaiO 1.00 .006 

100.00 



PETROGRAPHY OF THE IGNEOUS ROCKS. 49 

According to Lindgren/ the plagioclase in granodiorites is at least double the alkali feldspar 
and the latter ranges from 8 to 20 per cent. The porphyry of Brewery Hill is clearly mon- 
zonitic and not granodioritic. It is a little nearer to the intermediate type than much of the 
quartz monzonite porphyry mapped as the siUcic type on Plate I (in pocket). 

The quartz monzonite porphyry from the mouth of Browns Gulch is representative of the 
porphjny exposed along tliis gulch to its head and of much of that along both sides of the Swan. 
The rock is light gray and consists of conspicuous well-formed phenocrysts of orthoclase up to 
2 inches or so in length, and smaller ones of quartz in a fine-grained groundmass that is speckled 
with biotite and sparkles with minute disseminated crystals of pyrite. The orthoclase pheno- 
crysts are fairly clear and vitreous. The specimen illustrated in Plate VI, A(p, 44), while not 
from the same place in Browns Gulch, is representative of the rock here described. 

Under the microscope the rock, notwithstanding the development of pyrite, is seen to be 
not much altered. In addition to the large orthoclases, phenocrysts of plagioclase, quartz, 
biotite, apatite, and, rarely, allanite he in a fine-grained, evenly granular groundmass of quartz 
and orthoclase. The plagioclase is subhedral to anhedral and is andesine or oligoclase. Some 
of the crystals are partly altered to calcite, with probably some kaoUn. The quartz phenocrysts 
are rounded and embayed, lacking the fairly sharp bipyramidal form characteristic of them in 
much of the silicic type of porphyry. Although many of the biotite crystals are quite fresh, 
others are partly changed to chlorite. Apatite occurs in crystals large enough to be conspicuous 
among the microphenocrysts and attains lengths up to a millimeter. Allanite, the characteristic 
occurrence of which in these Colorado porphyries was first recognized by Iddings and Cross,' 
forms small stout prisms of the usual epidote habit, elongated parallel with the b axis and 
twinned on the orthopinacoid (100). The crystals of allanite in the rocks of the Breckenridge 
district are generally less than half a millimeter in length and maijy of them are minute anhedral 
grains recognizable as allanite only by their color, pleochroism, and index of refraction. It is 
rare that one thin section shows more than two or three crystals of allanite and some sections 
contain none of the mineral. 

The secondary minerals present are epidote, calcite, chlorite, and pyrite. The last has 
formed chiefly along cracks and on contact surfaces between other minerals. 

The norm of this rock, the basic constituents of pyrite and other secondary minerals being 
calculated as if in their original combinations, is as follows : 

Norm of quartz momonite porphyry from Browns Gulch. 

Quartz 24. 42 

Orthoclase: 23. 91 

Albite 30. 39 

Anort h i t e ] 3. 62 

Diopside 46 

Hyperathene 1. 82 

Magnetite 2. 09 

Ilmenite 61 

Apatite 34 

97.66 
Water 79 

98. 45 

This norm corresponds to toscanose. 

I 

> Lindgren, Waldemar, Oranodiorite and other Intermediate rocks: Am. Jour. Scl., 4th ser., toI. 9, 1000, p. 277. 

> Iddings, J. P., and Cross, Whitman, On the vridespread occurrence of allanite as an accessory constituent of many rocks: Am. Jour. Sci., 3d 

sen. vol. 30, 1885, pp. 108-111. Also Cross, Whitman, The laccolitlc mountain groups of Colorado, Utah, and Arisona: Fourteenth Ann. Rept. | 

U. S. Oeol. Survey, pt. 2, 1894, pp. 165-241, especially p. 223. , 

90047°— No. 75—11 4 



50 GEOLOGY AND ORE DEPOSITS OP BRECKENBIDGE DISTRICT, GOLOBADO. 

A preliminary calculation of the mode or actual mineralogical composition of the rock, the 
biotite being assumed to be the same as in the rock from Brewery Hill, gives the following result: 

Tentative mineralogical composition of quartz mxmzonite porphyry from Browns Gulch, 

Percent 
by weight. 

Quartz 25. 62 

Orthoclase 22. 80 

Albite 30.39 

Anorthite 13.34 

Biotite 1. 89 

Magnetite 2. 78 

Apatite 34 

Titanite 59 

Water, not used 72 



98.47 

In the course of the calculation it was found that about half of the ferrous oxide was in 
excess of the requirements of the mineralogical combinations used. This was calculated as 
magnetite. Under the sam^ supposition as was previously made regarding the feldspar mole- 
cules, the foregoing statement becomes — 

Mineralogical composition of quartz monzonite porphyry from Browns Gvlch. 

Quartz 25.62 

Orthoclase 24. 90 

Andesine ( Ab^gAnaj, or near AbjAni) 41. 63 

Biotite *. 1. 89 

Magnetite 2.78 

Apatite 34 

Titanite 59 

Water, not used in calculation 72 

Unaccounted for 1. 18 

99.65 

This is clearly the composition of a monzonitic rock. Although a more siliceous porphyry 
than that of Brewery Hill, containing less biotite and no hornblende, this rock has less quartz 
than the other, both in the norm and in the mode. 

MONZONITE PORPHYRY. 

MEGASCOPIC CHABACTEB. 

The calcic type of porphyry, represented by the mass of monzonite porphyry forming most 
of Bald Mountain and the greater portions of Nigger, Prospect, and Mineral hills, is generally 
a fine-grained rock of dull gray or green color. Where the rock is fresh the color is dark gray; 
but fresh material is nowhere exposed at the surface, and some shade of green, due to the 
secondary minerals chlorite and epidote, is eminently characteristic of this porphyry. Ordi- 
narily the color is dark grayish green, as may be seen along the many roads on both sides of 
French Gulch, west of Lincoln, or on the slopes of Bald Mountain. In underground workings, 
however, where the porphyry contains much finely disseminated pyrite, the tint may be much 
lighter. The general texture and appearance of the monzonite porphyry, as seen in hand 
specimens, is shown in Plate VIII. 

The porphyritic texture is nowhere conspicuous, few of the phenocrysts exceeding 5 milli- 
meters in length, and some varieties are almost aphanitic. The most common and noticeable 
of the porphyritic constituents is generally hornblende, but some varieties contain considerable 
biotite, and in a few facies this mineral is more abundant as phenocrysts than the amphibole. 
Both minerals in specimens collected from ordinary outcrops are as a rule lusterless and greenish 
in consequence of partial alteration to chlorite, epidote, and calcite. The feldspar phenocrysts, 



PROFESSIONAL PAPER Tb PLATE VI 




ONE MILE NORTHWEST OF SUMMIT OF MINERAL I 

Natural siie. Sea page 50. 




B. PROSPECT HILL, 700 FEET SOUTH OF ABUNDANCE SHAFT. 
grained variet/, tike the porphyry of the Wellmf^ton mine. Natural size. See pag 

MONZONITE PORPHYRY. 



PETBOGBAPHY OF THE IGNEOUS BOCKS. 



51 



invariably plagioclase, are uniformly small and inconspicuous and generally have the milk- 
white, dull appearance indicative of partial alteration. No quartz phenocrysts have been 
observed in this rock. 

IQCBOSCOFICAL CHABACTBB. 

Under the microscope in thin section (see fig. 3 and PI. IX, A and B) biotite on the whole 
appears to be fully as abundant among the phenocrysts as is hornblende. Perfectly fresh 
crystals are rare and the mineral, as seen in various specimens, exhibits all stages of alteration 
from a slight fringe of chloritization to complete replacement of the mineral by lamellar abro- 
gates of chlorite, epidote, and calcite, like those shown in Plate X, A and B. Quartz and 
muscovite or sericite have also been observed as alteration products of the biotite. The fresh 
mineral is a normal biotite with nothing noteworthy in color or optical behavior. Rather 
commonly the biotite forms phenocrystic aggregates, many of which are so crowded with grains 
of magnetite as to be nearly opaque and are surrounded by envelopes of granular coloriees 
augite. This change evidently took place 
before the rock had finally solidified. A 
part of the original biotite appears to have 
recrystallized as a biotitic aggregate while 
another part was transformed into mt^- 
netite, augite, and probably some ortho- 
clase, the last crystallizing in the ground- 
mass outside of the augitic envelope.' In 
the course of weathering the augite may 
be changed to calcite while the biotite still 
remains nearly fresh. 

The hornblende is generally some 
shade of brownish or yellowish green in 
thin section, the pleochroism being X, 
yellow; Y, greenish brown; and Z, brown- 
ish green to dark green. The absorption 
is Y >< Z > X. Sharp euhedral forms are 
rare, the crystals as a rule being more 
or less rounded prisms without terminal 
planes. Magmatic resorption with partial 
recrystallization of the unstable horn- 
blende molecule has taken place in some 
varieties of the porphyry but not in others. 
Dark semiopaque rims consisting chiefly 
of augite and magnetite are well shown in 
a tresh hypersthene-bearing dioritic facies of the porphyry from the Welhngton mine. (See 
PI. X.) Here the inner part of the envelope, in contact with the minutely irr^pilar surface 
of the unaltered hornblende crystal, is so thickly crowded with particles of magnetite as to be 
opaque. Tliis layer grades outward into a semiopaque brownish layer in which granules of 
augite are recognizable with some difficulty. This layer in turn is covered by a thin skin of 
clear augite granules, nearly free from magnetite and showing an increase in size from within 
out. It appears from this that the change undeigone by the hornblende material involves 
more than a single step. The dense layer of magnetite and pyroxene first formed itself under- 
goes some subsequent recrystalUzation and clarification. It is not certain, however, that this 
process alone proceeds so far as to produce the external envelope of clear augite. That is 
probably due to the reaction of calcic material from the magma with the ferruginous and mag- 
nesic constituents in the darker part of the envelope, resulting in the formation of clear augite. 




Froims a.— CJunctertallc 
acea ic thia section, with nl« 
eralucfde. p, PlBgloclase; b, 
b.boniblendF: tns, magnetlle, 



moQianlU porphyry (caklo type) &3 
I citisied. X 30. Drawn by aid of the om- 
iotite, partly allered to chlorlls and epidote: 
rith small crystals o[ npetlte; g.firDuiidmBss, 



< Pw Iddlngs, J. 1 



minerals, New York 



PLATE IX. 

Photomicrographs of Fresh and Altered Monzonite Porphyry. 

A. Monzonite porphyry, near diorite porphyry. Wellington mine. X 35. 

The large rounded phenocr>'^t is an aggregate of biotite, magnetite, and augite after hornblende. The rock 
is described with a chemical analysis on pages 55-56. Note the minute fissures, containing carbonate, traversing 
the otherwise fresh rock. 

B. Same with nicols crossed. 

C Same rock sericitized and carbonatized near the vein. Described on page 95, with chemical analyses. 

The dark-gray areas are chiefly siderite, which refracts the transmitted light so as to appear darker than reality 
in the photograph. The lighter areas are quartz and sericlte. X 40. 

52 








PROFESSIONAL PAPER 7 


PL«TI ,X 




i 


1 




i 




i 


1 


|p^';gag 


1 


^St 


i 


m 


pp 


m 




1 


m 


^^ai 


i 


ml 






PHOTOMICROGRAPHS OF FRESH AND ALTERED M0N20NITE PORPHYRY. 



PETROGRAPHY OF THE IGNEOUS ROCKS. 53 

Entirely fresh hornblende is rare in the calcic type of porphyry. Common alteration 
products are chlorite, calcite, epidote, and quartz. In many specimens the former presence of 
hornblende is shown only by the shape of these aggregates, every particle of the original mineiai 
having disappeared. 

Minerals of the pyroxene group are but scantily represented among the phenocrysts, 
although a nearly coloriess augite or diopside and a little hypersthene occur as microscopic 
phenocrysts in some of the freshest rock of the Wellington mine. Both minerals succumb 
readily to weathering and were probably once present in different parts of the porphyry masses 
where the femic constituents are now changed to chlorite, calcite, epidote, and other secondary 
products. The monocUnic pyroxene forms irregular anhedrons or phenocrystic aggregates. 
Hjrpersthene in some of the freshest rock of the Wellington mine forms short, stout, partly 
rounded prisms with the pale-green and red pleochroism characteristic of this mineral. The 
crystals generally show some alteration to bastit,e. 

As a rule the phenocrysts of the femic minerals are more or less intergrown with one another. 
The feldspar phenocrysts of the calcic type of porphyry are all plagioclase and determinations 
on combined albite and Carlsbad twins give compositions varying between AbjAnj and AbjAn,, 
corresponding to the part of the isomorphous feldspar series usually designated labradorite. 
The mineral has no special peculiarities in these rocks and except in some of the freshest speci- 
mens obtained from deep workings is mora or less altered. The common decomposition products 
are calcite, epidote, and sericite. Kaolin is found in some thin sections, but is not abimdant 
and its development is not a characteristic feature of the rock alteration. 

The groundmass of the calcic type of porphyry is holocrystalline. The constituent crys- 
talline grains vary so widely in shape and size that an estimate of coarseness or fineness of 
texture is difficult; but the average size of the grains is probably between 0.1 and 0.3 millimeter. 
The abundant feldspar laths give the groundmass a very different texture from that of the 
sificic type, as may be seen on comparing figures 2 (p. 44) and 3 (p. 51). 

The minerals composing the groundmass are plagioclase Oabradorite for the most part), 
orthoclase, quartz, biotite, augite, hypersthene, hornblende, magnetite, apatite, allanite, and 
zircon. The plagioclase is in general partly euhedral, but the orthoclase and quartz show no 
crystal outline and occupy the interstices between the plagioclase. In some thin sections the 
orthoclase appears as poikiUtic areas. More commonly it is intergrown with quartz as micro- 
pegmatite. 

As in the phenocrysts, the femic constituents of the groundmass are generally decomposed 
to chlorite, calcite, and epidote. Only in some of the freshest specimens obtained at a distance 
from the ore bodies is there any pyroxene remaining or are the biotite and hornblende entirely 
free from alteration. 

Among the accessory minerals allanite and titanite are rare and are not present in every 
thin section. 

CHEMICAL COMPOSITION AND CLASSIFICATION. 

The only samples of the calcic type of porphyry analyzed in the course of the present 
investigation were collected from the Wellington mine. These are perhaps slightly finer grained 
and a little more femic than the average of the rock mapped on Plate I (in pocket) as monzonite 
porphyry, being, in fact, diorite porphyries rather than monzonite porphyries; but they have 
the advantage of being almost perfectly fresh and were selected with the view to comparison 
with altered rock from the same mine. Inasmuch as many analyses of similar monzonite 
porphyries from the Leadville and Tenmile districts are available, the expenditure of additional 
chemical work on a thoroughly representative and of necessity more or less epidotized and 
chloritized specimen of the calcic type of porphyry from the Breckenridge district did not 
appear called for. 



PLATE X. 
Photomicrographs of Porphyries. 

A. Quartz monzonite porphyry (intermediate variety), east slope of Mount Guyot. X 40. 

Shows porphyritic alteration. The biotite is changed to aggregates of chlorite (ch), epidote (e), calcite (c), 
and quartz (q). The groundmaas, were the nicols crossed, would appear as a very fine mosaic of quartz and 
orthoclase. 

B. Quartz monzonite porphyry, slope east of Hoosier Pass. X 40. 

Rock is altered and contains much sericite as well as chlorite. The bent crystal of biotite, about two-thirds 
of which is shown, has been changed to chlorite, sericite, and calcite. 

C. Monzonite porphyry, near diorite porphjTy, Wellington mine. X 40. Nicols crossed. 

Photograph intended particularly to show the characteristic rimming of the hornblende pheuocrysts as 
described on page 51. 

54 



PROFESSIONAL P, 



^'^ 


^'■''^M 






IM 


M^---ii 





PHOTOMICROGRAPHS OF PORPHYRIES. 



PETROGRAPHY OF THE IGNEOUS ROCKS. 



55 



The two analyses made are as follows: 

Chemical analyses ofdiorire porphyry. 
[W. T. Schaller, analyst.] 



8iO,.. 
A1,0,. 
FejO,. 
FeO.. 
MgO.. 
CaO.. 
NaiO. 
KsO.. 
HaO- 
HjO+ 
TiOj.. 
ZrO,.. 
CO,... 
PjOa.. 
FeSs.. 



55.44 

14.95 

4.37 

5.18 

a58 

6.12 

4.44 

2.83 

.12 

.84 

1.22 

None. 

.35 

.49 

.09 



1 

57.35 


16.29 


a 15 


4.36 


2.41 


5.66 


4.50 


a39 


.15 


.70 


1.07 


Trace. 


.46 


.70 


.09 



Cr,Oi 

v,o, 

NIO, 

MnO 

BaO 

SrO 

LlOa 

ZnO 

Rare earths. 



Specific gravity of rock mass 

Specific gravity of rock powder at 25* C . 



0.22 

.16 

.04 

None. 



None. 

Trace. 

None. 

0.12 

.10 

.05 

None. 

None. 

None. 



100.44 



100.55 



2.827 
2.863 



2.763 
2.790 



1. Diorite porphyry. Andose (Br. 51). Wellington mine, dump of Extenuate tunnel. 

2. Diorite porphyry. Andose (Br. 217). Wellington mine, northeast end of Oro level. 

The sample used for analysis No. 1 is a hard, fresh dark-gray rock of splintery fracture, 
with glistening, black, prismatic phenocrysts of hornblende up to about 5 millimeters in diam- 
eter. As it was collected from the mine dump its exact locality is not certainly known, but it 
probably came from a crosscut, about 50 feet long, that runs northwest from the end of the drift 
on the Spur vein on the Extenuate level. (See PL XXVIII, p. 130.) This crosscut is all in 
fresh, hard country rock like the specimen. 

Under the microscope the phenocrysts of hornblende are seen to be accompanied by smaller 
phenocrysts of plagioclase, pale augite or diopside, hypersthene, biotite, magnetite, and apatite. 
Hornblende is the only mineral that is distinctively and solely a porphyritic constituent. The 
others occur also in the groundmass and such is the gradation in size that no definite distinction 
of groundmass from phenocrysts is possible ; in other words, the rock has a seriate porphyritic 
fabric.^ The preponderant constituents of the groundmass are lath-shaped plagioclases. With 
these are intersertal quartz and orthoclase, in part micrographically intergrown, and some 
intersertal grains of the femic minerals. The hornblende is of the ordinary brownish-green 
variety and is noteworthy only for the well-developed reaction rims described on page 51 and 
illustrated in Plate X, C (p. 54). The hypersthene, in stout subhedral prisms up to about a 
millimeter in length, is distinctly pleochroic in the usual faint red and green tints. Biotite is 
not abundant and occurs as tiny anhedral flakes, many of which are intergrown with the 
pyroxenes. The plagioclase is fresh and clear. Most of it is between AbjAnj and AbjAn, in 
composition. The orthoclase is anhedral, generally rather turbid, and is not a conspicuous 
constituent. A little zircon is present, but no allanite was seen. 

The norm of the rock is as follows : 

Norm of diorite porphyry (Br. 52) from Wellington mine. 

Quartz 3.00 

Orthoclase 16.68 

Albite 37.20 

Anorthite 12. 51 

Diopside 12. 33 

Hypersthene 6. 81 

Magnetite 6. 26 

Ilmenite 2. 28 

Apatite 1. 01 

Pyrite 09 

98.17 

Water 96 

COj 35 

99.48 

1 Iddlngs, J. P., Igneous rocks, vol. 1. New York and London, 1909, p. 197. 



56 GEOLOGY AND OBE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

According to this norm and the chemical analysis the rock is dosaUc, perfelic, alkalicalcic, 
and dosodic. It is therefore andose. 

The presence together of diopside, hypersthene, hornblende, and biotite, the composition 
of none of which is accurately known, renders calculation of the mode from the chemical analysis 
impracticable. In the actual rock part of the potash in the orthoclase of the norm must be 
combined in biotite and part of the lime in the normative diopside must be in the plagioclase, 
which is certainly more calcic than the AbgAni of the norm, optical determination on some of the 
albite Carlsbad twins giving Ab^oAn^ with maximum extinction angles of at least 36° in sections 
normal to (010). The norm sufficiently expresses the mineralogical composition of the rock to 
show that the ratio of orthoclase to plagioclase is less than that in typical monzonite. Conse- 
quently, although mapped with the monzonite porphyry of which it is an inseparable facies, 
this rock is petrographically a diorite porphyry. 

The rock of analysis No. 2 was selected for the particular purpose of comparison with altered 
varieties near by, as is shown on page 95. It is a dark-gray, compact, splintery rock with a few 
dull-black phenocrysts of hornblende in a nearly aphanitic groundmass. Under the micro- 
scope the apparent phenocrysts of hornblende are seen to be completely changed to aggregates 
of biotite crowded with particles of tnagnetite. These pseudomorphous phenocrysts, few of 
which exceed 3 miUimeters in length, are sparsely distributed through a groundmass consisting of 
a felted aggregate of subhedral to lath-shaped plagioclase with many small irregular flecks of 
biotite; with diopside in minute rounded prisms and larger anhedrons; with a good deal of clear 
interstitial orthoclase ; with a httle interstitial quartz ; and with grains of magnetite and small 
prisms of apatite. The plagioclase is mostly labradorite, crystals tested being near AbjAn, 
in composition, a fact which indicates that only a part of the soda is in this mineral. Although 
the constituents of the rock aie generally quite fresh, there is some carbonate, probably near 
siderite, present in microscopic cracks and in little bunches here and there where these cracks 
traverse minerals particularly susceptible to carbonatization. The pseudomorphs of biotite and 
magnetite after hornblende are generally surrounded by a little halo of this carbonate, which, 
as the chemical analysis indicates, composes about 1 per cent of the rock. 

The norm of this diorite porphyry is as follows: 

Norm of diorite porphyry (Br. 217) from the Oro level of the V^'dlington mine. 

Quartz 4. 50 

Orthoclase 20. 02 

Albite 1 37. 73 

Anorthite 14. 18 

Diopside 8.41 

Hypersthene 5.61 

Magnetite 4. 41 

Apatite 1.34 

Ilmenite 1. 98 

Pyrite 09 



98.27 
Water, etc., not used 1. 46 



99.73 

This norm, like that on page 55, corresponds to andose. Both norms are much alike, but 
it is noteworthy that while the one rock actually contains hypersthene the other, so far as the 
study of one thin section shows, does not. 



AN EXCEPTIONAL FACIES. 



A porphyry diflFering from any other observed in the district forms a small north-south 
dike on the steep spur east of Monitor Gulch, near Famcomb Hill. It is dark gray, with pheno- 
crysts of plagioclase up to 5 millimeters in length and smaller ones of biotite, in an aphanitic 
groundmass. The microscope shows phenocrysts of clear andesine (near AbjAnj) and fresh 
biotite in a fine-grained feldspathic groundmass that contains a good deal of secondary calcite 



PETROGRAPHY OF THE IGNEOUS ROCKS. 57 

and is chiefly remarkable for the abundant occurrence of biotite in wispy aggregates of very 
minute scales. These are distributed all through the groundmass and form little fringes or 
shells around the phenocrysts of the same mineral. Some of the aggregates have a form sug- 
gestive of pseudomorphous development after hornblende and the general habit of the mineral 
and its association with calcite are indicative of secondary origin. Biotite has been reported 
as an alteration product of olivine but is not one of the minerals ordinarily arising from rock 
decomposition. Unfortimately the Monitor Gulch rock does not furnish quite conclusive 
evidence that the mineral is secondary. The rock is a dioritic or andesitic porphyry belongiog 
probably to the calcic division of the Breckenridge porphyries. 

QUARTZ MONZONITE PORPHYRY (INTERMEDIATE TYPE). 

DI8TBIBT7TIOK. 

The intermediate type of porphyry has not been separately mapped on Plate I (in pocket). 
The reason for this is that porphyry of distinctly intermediate character forms no large masses 
within the area specially studied. Moreover, this type, if type it can be called, possesses no 
characteristic property so readily recognizable that consistent discrimination over the whole 
field would be possible without much more elaborate chemical and petrographic work than the 
prospective results would justify. The texture and appearance of some of the intermediate 
porphyry are shown in Plate XI. 

The intermediate variety is best exemplified by the thick sheet that forms the upper part 
of Mount Guyot, just east of the head of French Gulch and outside of the area mapped. Other 
occurrences are some intrusive sheets east of Hoosier Pass, 8 miles south of Breckenridge. 
Part of the mass mapped as monzonite porphyry at the mouth of Gibson Gulch, a mile east- 
northeast of Breckenridge, is of intermediate character, as is also some material thrown out 
from a shaft in the Mekka placer, on the south side of French Gulch. On the upper Swan a dike 
(shown in Plate I) just south of Snyder^s camp and bench mark 9823 is composed of the inter- 
mediate varietv, which forms a considerable mass also near the mouth of the middle Swan east 
of Georgia Gulch and outside of the mapped area. The porphyry on the west slope of Gibson 
Hill and that exposed along the railway between Rocky Point and Bacon have some inter- 
mediate characteristics but have been classed with the sihcic type. 

MEGASCOPIC FEATT7BES OF THE POBPHYBY OF MOUNT GT7YOT. 

The rock of Moimt Guyot was studied on the east slope of the peak, from Georgia Pass 
to the summit. In Georgia Pass the pre-Cambrian rocks and a Uttle mass of disturbed, much 
indurated dark shale and some quartzite are cut by dikes of a fresh dark-gray homblendic por- 
phyry wliich the microscope shows to be a rather femic variety of the calcic type. It is a diorite 
porphyry with unusually abundant augite (or diopside) in the groundmass. These dikes appear 
to be offshoots 'from the main Guyot mass of porphyry and to represent a femic marginal f acies 
of that rock. As the slope west of the pass is ascended, the prevalent variety of the porphyry 
is first a light-gray rock showing small phenocrysts of plagioclase and biotite and suggesting in 
the presence of hornblende and in the absence of noticeable quartz phenocrysts the calcic rather 
than the silicic type of porphyry. The average size of grain is apparently about 3 millimeters. 
The microscope, however, reveals unexpected resemblances to the siliceous quartz monzonite por- 
phyry. The thin sections show abundant subhedral to euhedral phenocrysts of plagioclase, 
biotite, and hornblende, with some rounded phenocrysts of quartz and grains of magnetite in a 
very fine granular quartz-orthoclase groundmass such as is found in the silicic type of porphyry. 
The plagioclase, as determined by extinctions of albite-Carlsbad twins, maximum extinctions in 
the zone perpendicular to (010), and the mean index of refraction, is near AbiAn^. The greenish 
yellow hornblende has the pleochroism X greenish yellow, Y yellowish green, and Z deep grass 
green. The absorption is Y> <Z>X. The biotite is the usual kirid present in these rocks. 
Small prisms of apatite are fairly abundant, but no allanite was detected in the single section 
examined. The secondary minerals, sparingly present, are epidote and chlorite. 



58 



GEOLOGY AND ORE DEPOSITS OP BaECKENBIDGE DISTRICT, COLORADO. 



Toward the summit of the peak the porphyry on the whole (see PI. XI) shows a scarcely 
perceptible gradation to a more equally granular rock, becoming at the top of the peak virtu- 
ally a fine-grained quartz monzonite with the hght^ray, very sUghtly reddish tint, common 
to these plagioclase-orthoclase rocks. 

MZCBOBCOPIOALr FEATUXIE8 OF THK POSFEZBY OF HOUITT OTTTOT. 
A thin section of the rock from the summit of Mount Guyot shows the texture iUustrat«d 
in figure 4. Although some of the plagioclase has a porphyritic development, the distinction 
between phenocrysts and groundmasa is almost lost, and the rock has nearly the equigranular 
texture of a plutonic mass. 

The constituents, which are almost ideally fresh, are plagioclase whose composition 

(AbiAn,) is on the line between andesine 
and labradorite, orthoclase, quartz, biotite, 
and hornblende in part intergrown, titanite, 
apatite, and magnetite. No allanite was 
seen in the thin section studied. 

The ]>lagioclase is fresh, and as it is 
sharply twinned according to the albite, 
Carlsbad, and perichne laws without much 
2onalstructure,itscharacter is closely deter- 
minable by oj)tical means. My own deter- 
minations gave as an average Abs,An„. 
Mr. F. C. Calkins kindly made some check 
determinations which gave a ratio of 
Aba,An„. The feldspar is thus virtually 
AbjAnt, or andesine. Where the crystab 
are zoned, the composition may range from 
Abg^jo to Ab^Uj,. The form of the 
plagioclase is subhedral to euhedral. The 
orthoclase and quartz are entirely anhedral 
and tend to be interstitial with reference 

JtomiK 1.-C<«™ly cryslalline quwti monxonlte porphyry (liitfnnedlaM tO theplagioclaSC. Thoorthoclase, although 
type), from the lummtt of Mount Guyot, aa seen In thin swtton with ijicob fresh, shoWS the USUal dustV turbiditV of 
cn»s«l. X 30, Drawn with camen hiclda. Texture nKirly cqulgraiiuliir. , . , , . . 

p, riBeh>diu«: q, qaaru; or, orthoctaae; b, biotite. that mineral, and some of it contains a 

httle perthitic albite. The biotite and 
hornblende differ ih no way from the occurrences of these two minerals in the other porphyries 
already described. 

CHEMICAL COHPOBZnON AND CLASaiFICATION OF THE PORFHYSY OF HDTTHT GtrrOT. 
The chemical composition of the porphyry from the top of Mount Guyot is as follows: 

Chemical anatyaU of quartz moruonilf porphyry/rom the tummil of Mount Guyot, 
|R.C. Wells, uifllyjl.] 




P,0,. 



AljO,-.- 
Fe,Oj.., 
PeO.... 
MgO.... 
CaO.... 
NajO... 
K,0.... 
H,0-. . 
H,0-|-. . 
TiO,.... 
ZrO,.... 



16.99 
2.59 
2.64 
1.13 
3.95 
3.78 
3.51 ! 



V,0,-- 
NiO... 
MnO.. 
BaO... 



.07 I LijO None. 



J, S. GEOLOGICAL SURVEY 



PROFESSIONAL PAPER 75 PLATE X 




A. SUMMIT OF MOUNT GUYOT. 

Natural size. Descfibad with chemical analyses on pages 57-59- 




B. RIDGE 2 MILES SOUTHWEST OF BOREAa 

Natural siie. See page 57. 

QUARTZ MONZONITE PORPHYRY, INTERMEDIATE VARIETIES 



f 



PETRgGRAPHY OF THE IGNEOUS EOCKg. 59 

The norm calculated from this analysis is as follows : 

Norm of quartz monzoniU porphyry from the summit of Mount Guyot. 

Percent 
by weight. 

Quartz 18. 31 

Orthoclaae 20. 57 

Albite 31. 96 

Anorthite 17.79 

Corundum 51 

Hypersthene 6. 04 

Apatite 67 

Ilmenite '. 91 

Magnetite 3. 71 

99.47 
Water 32 

99. 79 

Consequently in the American quantitative classification the rock is persalic, quardofeUc, 
alkalicalcic; sodipotassic but very near dosodic, and is therefore amiatose^ near yellowstonose. 

The mode may be calculated with the same data as regards the hornblende and biotite 
that were used on page 47 for the porphyry of Brewery Hill. This gives the following result: 

Tentative composition of quartz momonite porphyry from the summit of Mount Guyot. 

Per cent 
by weight. 

Quartz 20.74 

Orthoclaae 16.17 

Albite 32. 07 

Anorthite..: 15.88 

Biotite 8.16 

Hornblende 1. 95 

Apatite 67 

Titanite 59 

Magnetite 3.25 

99. 48 

In the light of the microscopical study the foregoing may be rearranged as follows : 

Mineraloffical composition of quartz monzonite porphyry from the summit of Mount Guyot. 

Per cent 
by weight. 

Quartz 20. 74 

Orthoclaae (with 29K2O and ISNa^O) 25. 63 

Andesine (AbjAuj) 38. 49 

Biotite 8.16 

Hornblende 1. 95 

Apatite 67 

Titanite 59 

Magnetite 3.25 



99.48 

This calculation, which is probably very close to the actual composition of the rock, shows 
that the orthoclase contains a considerable quantity of the albite molecule and suggests that 
in the calculated compositions on pages 48 and 50, an additional part of the albite should be 
transferred from the plagioclase to the alkali feldspar. 

OTHER FACIE8. 

The rock referred to on page 57 as occurring near the mouth of Gibson Gulch, and belonging 
probably with the intermediate division of the porphyries, is apparently merely a local facies 
of the calcic variety. As exposed at the mouth of the Johannesburg tunnel this is a light^gray, 
fine-granular rock in which are visible a very few small phenocrysts of hornblende. Under 



60 GEOLOGY AND OBE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO 

the microscope the general texture is seen to be subhedral granular and, although finer, 
resembles that of the Mount Guyot rock. The mineral constituents of the two are the same. 

The material tlirown out from the prospect shaft in the Mekka or Sisler placer is of two 
kinds. One is a fresh porphyry of the silicic type in wliich unusually numerous phenocrysts of 
andesine, quartz, orthoclase, and biotite are distinctly separable from a microgranular ground- 
mass of orthoclase and quartz having an average diameter of grain of about 0.1 millimeter. 
Small crystals of allanite, some sharply euhedral and twinned, are fairly abundant. This 
porphyry, which appears to be the general bedrock of the placer, is generally similar to the 
rock analyzed and described from Brewery Hill (pp. 45-48.) The rock associated with it, 
exactly in what relation is not known, is darker, is not noticeably porphyritic, and contains 
unusually abundant biotite. Under the microscope it shows a subhedral granular texture, the 
average diameter of grain being about 1 millimeter. The constituents are andesine (Ab5,An47), 
quartz, biotite, hornblende, magnetite, apatite, and zircon. Quartz is abundant. It fills 
interstices between and in part incloses the subhedral plagioclase. The most noteworthy 
feature of the rock is the absence of orthoclase, which in this district is almost inyariably 
present in the porphyries even in those in which quartz is less abimdant than in this facies, 
where the potassium apparently has gone into biotite instead of into alkali feldspar. The 
rock is a quartz-mica diorite. 

Another rock deserving mention, although it is not within the area mapped, is that of the 
intrusive mass that is cut through by the middle fork of the Swan, near its mouth. This is a 
fine-grained fresh pinkish-gray porphyry, with small sparse phenocrysts of hornblende and 
biotite, which in general appearance much resembles the variety from the mouth of Gibson 
Gulch, the similarity extending also to microscopical features. Save for the occasional pheno- 
crysts mentioned, which do not appear in every thin section, the rock is subhedral granular. 
The constituents are andesine (Ab^^An^o-Abj^An^g, with more sodic peripheral zones in some 
crystals), orthoclase, quartz, biotite, hornblende, diopside, magnetite, and apatite. The 
hornblende is intergrown with biotite and pyroxene, the pyroxene in such case forming the 
kernel of the crystal. The quartz and orthoclase are anhedral and are largely interstitial 
with reference to the plagioclase. This rock is so inconspicuously porphyritic that it is vir- 
tually a fine-grained quartz monzonite. 

RELATIONS OF THE PORPHYRIES TO ONE ANOTHER. 

The foregoing descriptions have made fairly clear the fact that all the porphyries of 
ihe district are closely related and that the types recognized correspond to parts of a generally 
continuous series. This is well brought out by the accompanying table, in which are assembled 
all the available chemical analyses of the comparatively fresh porphyries of the Leadville, 
Tenmile, and Breckenridge districts. The same general relationship is displayed graphically 
by the variation diagram of figure 5, constructed from the analyses in the table by plotting 
the molecular porportions of the principal constituents. In detail, however, the diagram 
shows that the proportions of the different constituents vary in an irregular way, analyses 
showing nearly the same silica, for example, exhibiting wide divergence in other constituents, 
so that the more closely spaced the analyses along the siUca abscissas the more irregular appear 
the curves connecting the ordinates of each base. Thus, it can not be said that the series of 
analyses represents uniform change in composition according to any simple set of laws. For 
example, a constituent neither increases regularly as another decreases nor closely parallels 
another in its variations from one rock fades to another. The most that can be said is that alumina 
and the alkalies remain fairly constant, with soda always in excess of potash, and that lime, 
magnesia, and the iron oxides generally decrease with increase of silica. While the rocks 
analyzed are undoubtedly closely related as regards their magmatic derivation, their varia- 
tions have not the simple character to be expected in samples taken along a single cross section 
of one well-differentiated igneous mass. 

In the table the molecular ratios of the main constituents are given below the percentage 
figures. 



PETBOGRAPHY OF THE IGNEOUS ROCKS. 



61 






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GEOLOGY AND ORE 



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CHAPTER IV. 

STRUCTURE OF THE DISTRICT. 

BROAD F£A.TI7RE8. 

In the latitude of Breckenridge the generally unmetamorphosed sediments, chiefly Creta- 
ceous, and the associated intrusive porphyries, probably of Tertiary age, form a north-south belt 
from 8 to 10 miles wide, bounded on the east by the pre-Cambrian rocks of the Front Range and 
on the west by the same ancient terrane, as exposed along the Tenmile Range. The rocks of this 
meridional belt, although much disturbed, have oti tJie whole an easteriy dip, and the general 
structure is accordingly monoclinal, the basal sediments on the west lapping up on the east 
slope of the Temnile axis and the whole series on the east being faulted down against the 
pre-Cambrian crystalline rocks, toward wliich they prevailingly dip. 

This dominant feature of the structure is illustrated dia^ammatically in the simplest way 
and without any attempt at fidelity to scale in figure G. The actual relations, as will presently be 
seen, have far greater complexity than is suggested by so crude a diagram. 

Sedimentary beds occur on the west side of the Tenmile Range, and a consideration of the 
relations of these beds to those of the Breckenridge belt shows that the latter are merely part 
of a wider tectonic block 
or unit whose western 
boundary is the Mosquito 
fault, described by S F 
Emmons in the Tenmile 
foHo.' From the Mos 
quito fault (shown m fig 
7) to Georgia Pass where 
the sediments and por nonu a.— Dl^nmllhutntlDg the gvnenl structure of that«ctoDU:beHlawbkliBraa«nrldtelli^ 

phyriea of the Brecken- 
ridge belt are succeeded eastward by pre-Cambrian rocks, the distance is from 12 to 13 miles. 
Kvidently the sediments of the Breckenridge area once extended across the line of the present 
TenmUe Range and were continuous with those of the Tenmile district. Whether, prior to their 
erosion, the beds swept uninterruptedly up over what is now the crest of the range until they 
reached the Mosquito fault, or whether they were stepped down by intervening faults, has not 
been determined. The Breckenridge block as a whole has been tilted to the east through an angle 
of about 20°, so that along its eastern edge the Upper Cretaceous shale has been brought down 
to juxtaposition witli the pre-Cambrian, and on the west the basal beds of the sedimentary 
series outcrop along the east flank of the TenmUe Range, from whose liigher slopes erosion has 
stripped them away. The eastern slope of that range, although scored by ravines and scalloped 
by glacial cirques, has not lost all resemblance to the old surface upon which the sediments 
were deposite<l, as may be seen on looking along the mountains from any commanding point 
on their flank. This feature is especially noticeable in views toward Hoosier Pass from the 
vicinity of Breckenridge, such as those shown in Plates IV, B (p. 18), and XIV, A (p. 70), for 
in these the I>edding of the sediments (not apparent in the illustrations) is seen to lie at approx- 
imately the same angle as the long even slope on the pre-Camhrian rocks to the right. On 
Quandary Peak and on the Nortli Star Mountains, also, are remnants of the "Sawatch" 
quartzite, wliich by their position show a close correspondence between the old sea floor upon 
wliich they accumulated and the general east slope of the Tenmile Range. 

1 TemnUa district special [olio <No, W). Cicol. Atlu L'. S., L'. S. Geol. Sumf . 1K98. 




.Sf^ ' 



GEOLOGY AND ORE DEPOSITS OF BEECKENBIDGE DISTRICT, COLOBADO. 

^ When the geologic structure near Breckenridge is examined in 
detail the conception of regular monoclinal structure requires con- 

:, siderable modification. Minor folds and faults must be taken into 

I- account, but the chief disturbing element is tlie intrusion of the 

; sediments and pre-Cambrian rocks by porphyries, which, though 

i predominantly sheets or sills, nevertheless depart very widely from 

I the regidarity of form displayed by the intrusion of similar magma 

' in the Tenmile district. 



MOSQUITO FAULT. 

The Mosquito fault, which limits the Breckenridge tectonic 
block on the west, lies 4 or 5 mUes outside of the area here studied. 
As described by S. F. Emmons in the Tenmile folio, it is remark- 
ably devious and varies in shade from place to place, so that along 
part of its course it is a normal dislocation and in another part it b 
a steep reverse fault. Emmons states that the initiation of the 
Mosquito fault, the uphft of the Mosquito or .Tenmile Range, and 
the folding of the sediments in the Tenmile district occurred after 
the intrusion of the porphyries. Inasmuch as the Hayden map of 
Colorado indicates that in the Gore Mountains, north of the Tenmile 
area, the Dakota rests directly on the so-called Archean and trans- 
gresses westward over the line of the Mosquito fault, "passing with- 
out a break from a floor of Archean to one of Mesozoic sediments," 
Emmons was inclined to assign the fault a pre-Cretaceous, probably 
Jurassic age. He points out, however, that this supposition would 
make the porphyries older than similar porphyries elsewhere in 
Colorado and that final determination of the age of the Mosquito 
fault would better be postponed until further studies can be made, 
particularly of the relation of the fault to the Dakota in the Gore 
Mountains. It was hoped that there would be opportunity during 
the course of the Breckenridge work to visit these mountains and 
to gain some light on tiiis question; but the season was fully occu- 
pied witli problems more directly germane to the investigation in 
hand. It is clear, liowever, that the porphyries of the Breckenridge 
district are younger than the Upper Cretaceous shale and there can 
be little question that the similar porphyries of the Tenmile area are 
of the same age as those of Breckenridge. This part of the Ilayden 
map, moreover, has proved to be so inaccurate that any detailed 
relations shown by it can have but little weight against opposing 
evidence. Finally, the Ilayden map as it stands indicates a distribu- 
tion of the Dakota in the Gore Mountains that is apparently not alto- 
gether incompatible with the view that the Mosquito fault cuts the 
Cretaceous beds. The continuation of the fault northward beyond 
the Tenmile-Breckenri<lge region, as Emmons truly remarks, invites 
much closer study. At present the evidence in favor of a late Creta- 
ceous or Tertiary age for the Mosquito fault is preponderant. 



SilHIl 



It has already been said that the Cretaceous beds of the Breck- 
enridge area are faulted down against tlie pre-Cambrian on the 
east. The contact hes generally east of the area here studied in 



STRUCTUEE OF THE DISTRICT. 65 

detail and has not been continuously traced. Apparently it is not due to a single fault but 
to a belt of complex faulting to which belong the poorly exposed irregular faults of various 
trends, which in the northeast corner of the mapped area (PL I, in pocket) separate the Upper 
Cretaceous shale, and apparently the porphyry also, from the pre-Cambrian. At Georgia 
Pass the mass of the shale is separated from the pre-Cambrian by the porphjrry of Mount 
Guyot, which has some appearance of having been erupted partly along the fault zone. At the 
mouth of the Middle Swan there is a similar relationship between shale, quartz monzonite por- 
phyry, and pre-Cambrian. Here, however, there is some shale between the porphyry and the 
pre-Cambrian gneiss, so that it is by no means clear that the fault fissure served as a channel 
for the porphyry magma. About a mile up the North Swan shale and porphyry are succeeded 
to the east by pre-Cambrian rocks, but the contact is not exposed. From this point the con- 
tact appears to run generally northwest to Snake River, but no satisfactory exposures of it were 
seen in the few places where it was crossed in reconnaissance. On the north side of the Snake, 
however, shale extends for about 7 miles above the mouth of the river, to a point about half a 
mile above the main forks. Here the shale is plainly faulted down against the pre-Cambrian 
gneiss, which rises as a cliff to the east of the soft sedimentary rock and marks an entire change 
of topography from the relatively low shale terrane with open stream valleys to bold ridges and 
deep ravines in the crystalline series. 

Although at Georgia Pass the pre-Cambrian rocks, with a little disturbed and indurated 
quartzite and shale of doubtful age, are cut by homblendic porphyry apparently belonging to 
the monzonitic series, it is not altogether clear that this homblendic porphyxy is an offshoot 
from the main mass of porphyry forming Mount Guyot nor that accordingly the faulting took 
place before or during the intrusion. Elsewhere, as in the vicinity of Muggins Gulch, such 
meager evidence as is available indicates that the porphyry, like the shale, is faulted against 
the pre-Cambrian and that consequently the faulting, in part, at least, was later than the main 
porphyry intrusions. This accords with Emmons's conclusions at Leadville and in the Tenmile 
district. 

GENERAL STRUCTURE OF THE DISTRICT. 

If the porphyry intrusions are for the present disregarded, the essential structural elements 
of the Breckenridge district are (I) the fundamental pre-Cambrian terrane as exposed along the 
flank of the Tenmile Range west of the Blue (see Pis. I, in pocket, and XII) and underlying the 
other formations in general; (2) the red *' Wyoming" formation resting directly on the pre- 
Cambrian with a northeast dip of about 30® and from a thickness bf perhaps 1,000 feet at the 
southern border of the district, thinning northwestward until it virtually disappears 3 miles 
north of Breckenridge; (3) the Dakota quartzite, 200 to 300 feet thick, stretching in a much 
broken and buckled band with prevailing low northeasterly dip from the middle of the south 
boundary of the mapped area to its northwest comer; (4) the Upper Cretaceous shale, over 3,000 
feet thick, overlying the Datoka and dipping at angles ranging from 20 to 66® to the east or 
northeast; and, finally, (5) the pre-Cambrian, brought to the surface again by faulting. 

The sediments have been intricately cut up and greatly disturbed or disrupted by the 
intrusion of the porphyries, chiefly in the form of very irregular sheets but also as dikes. In a 
rough way certain stratigraphic horizons of intrusion are recognizable. Thus, between the 
Dakota and the '* Wyoming '^ are the quartz monzonite porphyry masses of Rocky Point, of the 
spur between Breckenridge and French Creek, and of the west slope of Gibson Hill. All these 
are of the intermediate variety. In the Dakota or in the lower part of the Upper Cretaceous 
shale are the thick and very irregular sheets of monzonite porphyry that make up most of Bald 
Mountain and Nigger, Prospect, and Mineral hills. Finally, throughout the shale formation 
intrusions of the silicic variety of quartz monzonite porphyry abound. While these generally 
show a tendency to follow the bedding of the shale their great irregularity of shape is evident 
from Plate I. It is safe to say that fully half the area studied is underlain by intrusive porphyry. 
The forcible injection of so much molten material into the sediments has greatly affected their 
structure. Beds originally contiguous have been forced apart and widely separated; others 

90047*'— No. 75—11 6 



66 



GEOLOGY AND ORE DEPOSITS OF BEECKENRIDGE DISTRICT, COLORADO. 



have been broken up and their fragments dispersed as inclusions through the magma; still others 
have been folded, crumpled, tilted, or faulted in the process of intrusion. The character of 
some of the intrusions is illustrated in miniature by figure 8, which is a sketch of a small siU 
intrusive in the partly shaly beds of the Dakota and exposed on the steep slope just northeast 
of Lincoln. The result of the porphyry intrusions is the production of a structure that can not 
be interpreted in detail. Even did the district afford satisfactorily full data regarding the sur- 
face relations of the rocks, as is far from being the case, deduction from these data of the actual 
underground structure would be impossible. The problem of determining the underground 
shape of such intrusive masses from a few surface observations may be likened to an attempt to 
ascertain the outlines of the porphyry areas north and south of the Swan on the basis of a few 
observations confined to the banks of that stream. These difficulties are dwelt on and emphar 
sized in order that the reader should not credit the largely hypothetical sections presented in 
Plate XII with more detailed accuracy than they really possess. Their construction was an 
aid in forming a conception of the general structure of the district, and they may prove helpful 

and suggestive to the reader 
who remembers that they 
contain many inferences and 
are much generalized b&low 
the profile representing the 
surface. 




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DAKOTA OVERIiAP. 

The disappearance of the 
pre-Dakota sediments along 
the east side of the Tenmile 
Range from Breckenridge 
northward is a feature so 
closely related to the struc- 
ture of the district as to find 
appropriate consideration 

Figure 8.— Diagrammatic sketch of a small sill of quarts monzonlte porphyry intrusive into ViA*.« T4- yvio^tKa T.^Tvt^^rv^U^T.t^A 

shale and quartilte on slope northeast of Ltacoln. ^®^®- ^^ ™^y ^f rememoerea, 

from the descriptions in pre- 
vious chapters, that beds ranging from the Cambrian to the Triassic, with a total thickness near 
Hoosier Pass that is probably somewhere between 5,000 and 10,000 feet, wholly disappear in a 
distance of about 10 miles northward along the Blue, so that the Dakota 2 miles nortli of Breck- 
enridge rests on the pre-Cambrian. There is also a rapid thinning out of part of the same beds 
from west to east near the latitude of Breckenridge; for, according to Emmons, there are ia the 
Tenmile district nearly 5,000 feet of pre-Dakota sediments, while near Breckenridge not over 
700 feet of this series is represented, and 2 miles north of that town all has vanished. Emmons 
has suggested in the Tenmile folio that the Mosquito uplift took place in Jurassic time and that 
the absence of the ** Wyoming '' and older sediments along the lower Blue may be due to an 
unconformity thus produced at the base of the Dakota. It has been shown, however, on 
page 43, that the porphyries of the Tenmile district are probably Tertiary, or at least late Cre- 
taceous, and if, as Emmons states, these porphyries are older than the Mosquito fault, then the 
uplift of the present Mosquito or Tenmile Range can not be the cause of the Dakota overlap. 
Moreover, if the deposition of the Dakota followed so sharp a local disturbance as the folding 
of the ''Wyoming" and associated beds in the Tenmile area and the vigorous uplift of the range, 
then apparently there should be indubitable evidence of angular unconformity between the 
Dakota and the ''Wyoming.'^ So far as the work at Breckenridge shows, there is no such dis- 
cordance, and the Dakota and Upper Cretaceous shale, as well as the older sediments, have been 
involved in the one great orogenic movement that has left a conspicuous mark on the region. 



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STBUCTUBE OF THE DISTRICT. 67 

This movement accordingly took place in late Cretaceous or Tertiary time. Finally, it is to be 
remembered that the Dakota is not the only formation that overlaps the pre-Cambrian, although 
it does so more conspicuously than the older formations. At Hoosier Pass the Cambrian, 
Silurian, Devonian, and Carboniferous are all represented, while in Illinois Gulch near Breck- 
en ridge the *' Wyoming" beds rest directly on the pre-Cambrian. Thus, even if it were possible 
to explain the Dakota overlap by the Mosquito uplift in Jurassic time, the ^* Wyoming" overlap 
and probably yet older overlaps below it would still remain to be accounted for. 

As Emmons* has stated, the sites of the present Sa watch and Colorado or Front ranges 
were probably occupied by land areas throughout the Paleozoic era. The pre-Cretaceous beds 
contain much coarse material and were evidently derived from no very distant masses of dis- 
integrating pre-Cambrian rocks. In many places they are clearly Uttoral deposits. It appears 
probable that the progressive overlap of the Paleozoic formations, culminating in the wide 
sweep of the Dakota, is the record -of a gradual though probably not uniform transgression of 
the sea over the pre-Cambrian land areas. A large region north of Breckenridge and east of 
the present crest of the Tenmile Range, embracing the crest of the Front Range, was probably 
never covered by the Paleozoic sea and was submerged only when the Dakota mantled its worn 
surface. From Breckenridge south to Hoosier Pass a slope ranging from 1 in 10, or about 5°, 
to 1 in 5, or about 10°, would have permitted the accumulation of 5,000 to 10,000 feet of sedi- 
ments at the site of the pass before the advancing sea began to deposit material where Brecken- 
ridge now stands. A similar gradient to the west, of 10° or less, would account for the difference 
in sedimentation in the Breckenridge and Tenmile districts. The comparatively steep offshore 
slope in this direction, resulting in the heavy accumulation of sediments just west of the present 
Tenmile Range, may have been an important factor in the initiation of the Mosquito fault and 
the uplift of the mountain ridge. 

It is not supposed that the sea encroached steadily upon the land throughout Paleozoic 
and early Mesozoic time. There were probably many oscillations and it is known that else- 
where in Colorado (see p. 30) distinct unconformities intervene between the Permian and 
Triassic beds and between the Maroon and Gunnison formations. It is quite possible that the 
movements which produced these unconformities affected the Breckenridge region also. 
Their influence, however, has not been definitely recognized. 

One economically important consequence of the overlap is the absence from the Brecken- 
ridge district of the ore-bearing limestones of Leadville. 

STRUCTURAIi DETAILiS. 

Flan of description. — The rough outline of the structure already presented requires filling 
in before it can be accepted as a reasonably satisfactory picture of the whole. This perhaps 
may be most clearly and systematicaUy accomplished by taking up in order the sections shown 
in Plate XII (p. 66) and commenting on the structural relations existing in the vicinity of each. 

Section A-A\ — In section A-A' the pre-Cambrian basement as exposed in North Barton 
Gulch appears on the left. Upon this rests the Dakota, which covers a considerable area 
northwest of Braddocks. The *' Wyoming'' here is lacking or is represented merely by a few 
reddish pebbly beds and a little dark-red shale not clearly separable, according to Mr. Bastin, 
from the basal part of the Dakota. 

The structure of the quartzite, owing to poor exposures and the thickness of the beds, is 
obscure and there is no place where its thickness is measurable. The beds are probably gently 
folded and perhaps faulted. The structure shown in the section is generalized and largely 
hypothetical. Overlying the quartzite with a dip of 30° to 40° is Upper Cretaceous shale, 
which here forms most of the bedrock beneath the auriferous gravel in the valley of the Blue. 
Between the Blue and the Swan is a low hill carved from an irregular intrusive mass of quartz 

1 Geology and mining industry of Leadville, Colo.: Mon. U. 8. Oeol. Survey, vol. U, 1886, pp. 21-dO. 



68 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

monzonite porphyry, then another narrower belt of shale in the bed of the Swan, succeeded by 
the very large body of quartz monzonite porphyry northeast of Valdoro. In the xnain this is 
probably a thick irregular intrusive sheet which is known to include many disrupted fragments 
and masses of shale. There is nothing to show, however, that the porphyry does not in some 
places continue downward across the stratification to its magmatic source. 

Section B-B\ — Section B-B' shows on the west of the Blue the terrace gravels between 
Barton and Cucumber gulches resting on the pre-Cambrian. It is possible, however, that 
some of the Dakota quartzite of Cucumber Gulch (see Plate I, in pocket) may extend as far 
north as the line of the section. Here the channel of the Blue is excavated in the pre-Cambrian, 
as shown by exposures along its banks and by drill holes. East of the Blue the ''Wyoming" 
formation is represented by some beds of conglomerate succeeded by red sandstone and shale. 
The basal beds are exposed in the bottoms of some of the old hydraulic workings along this side 
of the Blue, but the upper beds are concealed by terrace gravel and soil. The thickness of the 
''Wyoming" on the line of section B-B', provided no faulting has affected it, is from 500 to 700 
feet. This is rather remarkable in view of the virtual absence of the formation in the vicinity 
of the Barton gulches and suggests some concealed faulting. Overlying the "Wyoming" 
beds is a great intrusive mass of porphyry which on the supposition that it has the general 
shape of a sill is fully 1,300 feet thick. It terminates abruptly at both ends. At the north 
some intrusive contacts are exposed, the porphyry cutting quartzite, calcareous gray shale, and 
reddish shale without perceptible metamorphism. At the south the relations of the porphyry 
to the pre-Cambrian and Dakota are very obscure. Numerous prospect pits in this vicinity 
show a peculiar decomposed breccia consisting chiefly of porphyry fragments and partly rounded 
bowlders but including also some blocks of pre-Cambrian material. The large fragments are 
held in a light-gray matrix which appears to consist mainly of triturated porphyry and has the 
general aspect of a tuff. The origin of this material is a puzzling problem. It appears to imder- 
lie the soil and surface detritus over an area of nearly a quarter of a square mile and can hardly 
be a fault breccia. There is no evidence elsewhere in the district of local explosive volcanic 
activity. The most plausible explanation appears to be that the south end of the porphyry 
mass was intruded in an extremely viscous condition and was brecciated and triturated by the 
motion of its own intrusion. This is not altogether a satisfactory suggestion, but in the absence 
of good exposures of the material it is all that can be offered. The breccia is mapped as quartz 
monzonite porphyry on Plate I and occupies that part of the porphyry area l^^ing south of the 
more northerly of the two roads shown crossing the western slope of Gibson Hill. 

Overlying the porphyry and forming the crest of the north spur of Gibson Hill is the Dakota 
quartzite, which strikes from north to north-northwest and dips east at angles ranging from 
30° to 40°. Unless there is some faulting or concealed folding this dip requires a much greater 
thickness in the Dakota than is probably attained by that formation. Consequently as a sug- 
gestion of what may be the actuail structure two hypothetical faults are introduced in the 
section. The quartzite is followed in normal sequence to the northeast by a thick belt of Upper 
Cretaceous shale, which is the prevailing bedrock of the Gold Run placers. As is often the case, 
the natural exposures of tliis relatively soft rock are exceedingly unsatisfactory, and in mapping 
much dependence must be placed on prospect pits and placer workings. On the northeast side 
of Gold Run the black shale, as shown by the workings of the Jessie mine, extends in part under 
the same very irregular mass of porphyry that forms the hills east of Valdoro. Between Gold 
Run and the pre-Cambrian area in the northeast corner of the district the section crosses many 
alternations of porphyry and shale. Some of the shale masses are indubitably inclusions wholly 
surrounded by the igneous rock; others perhaps are connected with the main body of the shale. 
Finally, at the northeast end of the section shale and porphyry are shown faulted down against 
the pre-Cambrian. The fault plane is nowhere exposed, but dislocation is inferred from the 
fact that the shale maintains its northeasterly dip close up to the pre-Cambrian rocks. The 
Dakota, moreover is absent, and the shale in this part of the district is stratigraphicahy some 
thousands of feet at least above the base of the Cretaceous. 



U. 8. QEOLOOICAL 5URV 


EY 
















PROFESSIONAL P*P£H 75 PLATE XIII 








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The camera was set on the surface of the valley train. The willows in the foreground are growing on recent alluvium 
in the trench of the present stream. Bevond them is the valley-train terrace, approiimately on a level with the 
camera, and behind that is the older bencn formed by the terrace gravels. See page 73. 




. MILE NORTH OF 



STRUCTURE OF THE DISTRICT. 69 

Section C-C\ — Section C-C' shows at its southwest end some morainal material and 
terrace gravel resting on rock which is nowhere exposed near by but which is probably in part 
pre-Cambrian. Near Shock Hill is some Upper Cretaceous shale, with an unusual local dip 
to the west, overlain and perhaps irregularly cut by monzonite porphyry. The structure at 
Shock Hill is apparently a shallow syncline opening to the south. The Dakota is probably 
the bedrock of the Blue along this section and continues over to French Creek. There is no 
suggestion of close folding and the beds are probably gently flexed, as indicated in the section. 
Wliether or not they are also faulted is unknown. There is a little purplish-red shale on the 
northeast side of Shock Hill which is interbedded with the normal Dakota quartzite. 

On the northeast side of French Gulch the pre-Cambrian makes its appearance, and it is 
necessary to conclude that the Dakota is brought against the older terrane by a fault con- 
cealed by the gravels of French Gulch. The same or another fault is necessary also to accoimt 
for the relation between the "Wyoming" formation and the pre-Cambrian on the west slope 
of Gibson Hill, just north of the line of the section. At the base of the hill "Wyoming" beds 
with a dip to the east rest on the pre-Cambrian, but, as shown on Plate I (in pocket), the 
pre-Cambrian reappears much higher up the slope. The position and trend of this fault can 
not be ascertained on account of the extensive cloaking of this part of the district by gravels. 
As the section is followed up the slope of Gibson Hill the pre-Cambrian is succeeded by the 
Dakota. The exposures here are exceedingly poor, but the course of the contact, the absence 
of recognizable "Wyoming" beds, and the fact that the quartzite and associated shale dip 
toward the pre-Cambrian in the one or two places where observations could be made, all indi- 
cate that the pre-Cambrian is bounded on this side abo by a fault, as suggested in the section. 
The structure of the quartzite on Gibson Hill is obscure. Numerous tunnels and shafts enter 
the hill, but no work was in progress in any of them during 1909 and they are generaUy inac- 
cessible. The dumps^ of some show considerable red shaly material, similar to that on Shock 
Hill, which apparently is interbedded with the quartzite. The structure as represented in 
the section rests upon a very slender basis of fact. On the east end the quartzite is separated 
from the Upper Cretaceous shale by a dike of quartz monzonite porphyry that apparently 
occupies a fissure along which some faulting has taken place, the shale having been dropped 
against the quartzite for a distance of '200 or 300 feet. East of the shale lies a great mass 
of monzonite porphyry, which is the principal rock of Prospect and Mineral hills and was prob- 
ably once continuous with the porphyry of Nigger Hill and Bald Mountain. The sheetlike 
character of the intrusion is here scarcely recognizable, and the monzonite porphyry, as may 
be seen from Plate I, cuts the sedimentary rocks most irregularly in the vicinity of Gibson 
and Prospect gulches. This porphyry, which includes some masses of shale, continues across 
Gold Run and is then succeeded by the quartz monzonite porphyry of the great irregular sheet 
with dike apophyses already described in connection with sections A- A' and B-B'. Exposures 
throw no direct light on the character of the contact between the two porphyries. From 
evidence presented on page 71 it is supposed that the quartz monzonite porphyry cuts the 
monzonite porphyry, and this relation is indicated in the section by a conventional jagged 
line. Northeast of this contact the section cosses a small mass of monzonite porphyry thought 
to be an inclusion in the more siliceous eruptive. After passing through many masses of 
shale, probably for the most part ragged inclusions caught up in the magma, the plane of the 
section crosses the Swan, goes through a thick division of the Upper Cretaceous shale, and 
ends in the pre-Cambrian, against which the shale is faulted, as described in connection with 
section B-B'. 

Section D-D\ — Section D-D' shows on the left the morainal deposits south of Breckenridge 
resting in part on the pre-Cambrian. Somewhere in the space left blank in the section west 
of the Blue there is probably a considerable thickness of the red "Wyoming" beds, but nothing 
is positively known concerning the character of the bedrock beneath this part of the moraine. 
East of the Blue the section crosses the quartzite, shale, and porphyry of the vicinity of the 
Puzzle mine and Little Mountain. The structure here is more complex than can be shown in 



70 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

the section. The monzonite porphyry cuts the quartzite irregularly and has also forced it3 
way as sheets between the beds, as may be seen in the Puzzle-Gold Dust mine. Between the 
quartzite and shale along Dry Gulch there is apparently a fault whose fissure is occupied by 
the Gold Dust vein. On the north side of Illinois Gulch a lessees' tunnel on the Dunkin ground 
goes through about 200 feet of monzonite porphyry and then passes into black shale, in which 
it continues for several hundred feet to the face, as it was in 1909, although all the upper 
workings on the Dunkin group of claims on Nigger Hill are in the porphyry. The same shale 
with some beds of compact gray limestone underlies the porphyry at the middle railway bridge 
over Illinois Gulch. Here the base of the porphyry conforms to the bedding of the shale and 
dips to the north at about 35°. It appears that the porphyry of Nigger Hill is a thick uneven 
sheet resting in part on shale. In some places the lower surface conforms to the bedding, in 
others it cuts across the beds at various angles, and in still others it may continue down through 
the shale to great depth. In the section an attempt is made to generalize these relations. The 
chances are that most shafts on Nigger Hill, if sunk deep enough, would go through the por- 
phyry into quartzite or shale. The various masses of shale and quartzite crossed by the section 
between Nigger Hill and French Gulch are possibly inclusions in the porphyry; but the struc- 
tural relations seen in the Country Boy and Helen tunnels indicate that the larger bodies are 
probably continuous with the main mass of sediments underlying the greater part of the por- 
phyry. The tunnels mentioned reveal far greater complexity than can be shown in a section 
of this scale. It is evident that the lower part of the porphyry spUts into a number of sills 
and that faulting has greatly complicated the structural details. Under the gravels of French 
Creek, where crossed by the section, are Dakota quartzite and Upper Cretaceous shale, some 
of the black shale appearing on the north side of the creek at the base of Mineral Hill. The 
main mass of Mineral Hill is monzonite porphyry, which apparently is an irregular sill about 
1,200 feet thick at this place. The areal relations indicate that it is a part of the same sill 
that forms Bald Mountain. If so, it has been upUfted relatively to the Bald Mountain mass 
by faults along French Creek. Northeast of Lincoln Park the underground relations between 
the porphyries and shale, as indicated in the section, are almost wholly hypothetical. 

Section E-E'. — On the left section E-E' begins in morainal material, which probably 
covers "Wyoming" beds. Above these is a thick sheet of quartz monzonite porphyry of the 
intermediate variety, which has been intruded between the "Wyoming" and the Dakota. 
The bedding of the Dakota is not shown at this point. It was presumably much disturbed by 
the force of the intrusion. Northeast of this is considerably disturbed Upper Cretaceous shale, 
cut by a mass of monzonite porphyry that apparently is too irregular to be a sill. Northeast 
of this in turn is Dakota quartzite resting on "Wyoming" beds and separated from a part of 
the same formation by a fault that is nowhere actually exposed, although the general character 
of the contact leaves little doubt of its presence. Between the fault and Bald Mountain the 
section crosses a small irregular dome or quaquaversal, which results in the exposure of a small 
area of pre-Cambrian in the upper part of Illinois Gulch. (See PL I, in pocket.) On the east 
side of this pre-Cambrian the gently dipping beds of the "Wyoming" formation have a thick- 
ness of about 1,000 feet and are overlain by some of the basal beds of the Dakota, covered in 
turn by the great monzonite porphyry sill of Bald Mountain. This sill appears to have been 
intruded along the bedding planes of the Dakota, and, as shown on Plate I, fragmentary masses 
of strata referable to that formation occur as inclusions in the sill or rest upon its upper surface 
at several places overlooking French Gulch. Crossing the Upper Cretaceous shale belt of French 
Gulch the section passes through the intrusive quartz monzonite porphyry mass of Farncomb 
Hill. This is a highly irregular body which has sent out both dikes and sills into the surrounding 
shales. 

Section F-F\ — Section F-F' also crosses part of the quaquaversal uplift. The folds shown 
in the "Wyoming" are an attempt to generaUze from the data of a few poor exposures. The 
depth and form of the pre-Cambrian surface are conjectural. 

Section G-G\ — Section G-G', which differs from the others in being nearly meridional, is of 
interest as showing the extent of the great Swan River mass of quartz monzonite porphyry and 



S. GEOLOGICAL SURVEY 




A. VIEW SOUTH UP THE VALLEY OF THE BLUE FROM THE ROADSIDE THREE-QUARTERS OF 
A MILE NORTH OF BRECKENRIDGE. 

il bowldar.sWown surface of the valley train. On the right is Shock Hill. The distant 
k is Red Mountam, east of Hoosier Pass. See page 79. 



e foreground i! 



iarly Ic 




B. TERRACE GRAVELS WflTH BANNER HYDRAULIC WORKINGS ON THE WEST SIDE OF THE BLUE. 

The vi«« is from Cbson Hrll near the abandoned Kellogg mme, In the distance 19 the Tenmila Range To the left of 
the left of the mouth of Cucumber Gulch belong to the Iron 



STBUCTXJEE OF THE DISTRICT. 71 

the relation of the monzonite porphyry bodies of Mineral Hill and Bald Mountain. The French 
Creek tunnel, indicated on this section, shows clearly that the sedimentary beds, at this place 
at least, virtually pass entirely under the monzonite porphyry. The structure visible in this 
tunnel can not all be shown on the scale of Plate XII (p. 66). A more detailed section is given 
in figure 15, on page 138. The lowest beds, exposed at the face of the tunnel, are gray, mica- 
ceous, arkosic grits with disseminated pyrite. These sandstones, partly pebbly and unques- 
tionably belonging to the ^* Wyoming" formation, although they are not red, are overlain by 
reddish shale identical with that cut in the Gold Bell tunnel on Bald Mountain. This shale 
probably belongs at the top of the *' Wyoming,'' although it is possibly in the Morrison forma- 
tion. A little quartzite, supposedly Dakota, occurs near the portal of the tunnel. 

REIiATIVE AGES OF THE PORPHYRIES. 

The question whether the monzonite porphyry cuts the quartz monzonite porphyry, or vice 
versa, is one not easily answered. The two rocks, so far as exposures in this district are con- 
cerned, do not grade into each other, although there are varieties intermediate between the two 
extreme types. Where one porphyry comes against the other, the contact as a rule is as defi- 
nite as between other rocks distinguished in mapping. Nowhere in the district, however, so 
far as known, is there a place where the actual contact between fresh rocks of both kinds is 
exposed. On Prospect and Mineral hills and in the workings of the Wellington mine the quartz 
monzonite porphyry appears to cut the monzonite porphyry, or diorite porphyry, as dikes. On 
the other hand, as Plate I shows, there are a few masses of the more femic variety inclosed in 
the quartz monzonite porphyry. Are these small intrusive bodies or are they inclusions? 
There appears to be no practicable way of determining. Many of them are accompanied by 
shale, and the probabiUty is that, like the shale, they are inclusions caught up in the more silicic 
rock. On the whole, while the porphyries were probably erupted close together in time, it is 
believed that the monzonite porphyry is older than the quartz monzonite porphyry. 



CHAPTER V. 
QUATERNARY DEPOSITS. 

INTRODUCTORY STATEMENT. 

The Quaternary deposits in the Breckenridge district may in part be divided into glacial 
accumulations of Pleistocene age and stream gravels of the Recent epoch. During the Pleisto- 
cene there were two distinct cycles of advance and retreat and each has left a depositional 
record. The earlier is represented by what may conveniently be designated terrace gravels 
and possibly by what will be called the older hillside wash; the later by moraines and low-level 
gravels. These various deposits will be described in the general order of age. 

TERRACE GRAVELS. 

DISTBIBUnON. 

The terrace gravels occur along both sides of the valley of the Blue from a point about a 
mile south of Breckenridge northward to Dickey, a distance of 7J miles, and perhaps beyond 
Dillon, some gravel being reported on the west side of the Blue 15 miles north of Breckenridge. 
Their principal development, however, is within the boundaries of the district, where, as shown 
in Plate XIII, their terrace character is more evident in the landscape than on a map with 
so large a contour interval as 50 feet. At the latitude of Breckenridge the old fluvial gravel 
plain, of which the terrace deposits are renmants, must have had a width of 2i miles. It was 
probably wide also in the vicinity of Braddocks and may have extended over the area now 
known as Delaware Flats up the Swan as far as Galena Gulch. North of the junction of the . 
Blue and the Swan, however, the valley narrows, and although it opens again toward Dickey 
the constriction appears to be the northern limit of the important areas of terrace gravel. The 
greatest height to which the gravel extends is not easily determined, as the material generally 
merges with the local wash from the slopes back of the terrace. ' Southwest and west of Breck- 
enridge the terrace gravel appears to attain an elevation of 10,250 feet, or 650 feet above the 
Blue. On the spur east of Breckenridge over which passes the road to French Gulch the gravel 
reaches an elevation of at least 10,000 feet, or 250 feet above the nearest point on French Creek. 
On the spur south of Braddocks the upper limit of the gravel is at about 9,600 feet and north- 
west of Valdoro at 9,400 feet, or 250 feet above the point where the Blue and Swan unite. 

THICKNESS. 

The base of the terrace deposits is probably everywhere above the channels of the streams 
that now dissect them, although in many places no rock is exposed between the terrace gravels 
and the low-level gravels. The important sheet of terrace gravel east of Breckenridge, for 
example, merges with the low-level gravel along the Blue and along French Creek, except at 
the northwest end of the spur, where the higher gravel rests on quartzite about 50 feet above 
the present streams. Along the east side of the Blue, from French Gulch to Braddocks, the 
base of the terrace gravels is in most places at about the same height above the present flat 
floor of the valley. On the west side of the river there is less regularity in the altitude of the 
base of the gravels, and it is clear that here the surface upon which they were deposited was 
uneven. The old valley was broad and was diversified by low hills and spurs against whose 
slopes the gravels accumulated, in some places to the extent of complete burial. Shock Hill, 
for example, at one stage of valley aggradation must have stood out as a rocky island in a 
gravelly plain. Under these circumstances the maximum or average thickness of the gravel is 
72 



J. & QEOLOQICAL SURVEY 



PROFESSIONAL P: 




A. TERRACE GRAVELS OF THE BANNER PLACER, 1 MILE NORTH OF BRECKENRIDGE. 

Rounded gravels in the lower layer* (partly covered by loose detritus). Above these are some sandy layers and then 
coarse lubangular material. See page 73. 




The notebook la 8 Inches long. A c 



a toot to the right of the book. 



QUATEBNARY DEPOSITS. 73 

not easily estimated. Along the west base of Gibson Hill the deposits are probably nowhere 
over 50 feet thick, and they thin out on a rising floor to the east. Between Breckenridge and 
French Creek the terrace gravels have been extensively worked by the hydraulic method and 
the old pits show thicknesses ranging from 10 to 20 feet, so that these gravels constitute a 
comparatively thin layer over slopes very similar to those of the present surface. Due west 
of Breckenridge the old hydrauUc workings south of Shock Hill show a maximum thickness of 
about 50 feet. Between Shock Hill and Barton Gulch, in the Banner hydrauUc workings (see 
Pis. XIV and XV), the banks of gravel are from 75 to 100 feet high, this being probably the 
thickest deposit of terrace gravel exposed in the district, although it is by no means certain 
that shafts or borings would not reveal greater thicknesses in some of the large masses west 
of the Blue. It is reported that a hole 250 feet deep was drilled in the terrace gravel west of 
Breckenridge without reaching solid rock. The exact position of this hole was not ascer- 
tained, however, and it possibly went through some morainal material as well as terrace gravel. 

CHARACTER OF MATERIAL. 

The bowlders in the typical terrace gravel are well roimded and range up to 4 feet in diam- 
eter, although those over 2 feet are uncommon. They consist of various granitic and schistose 
rocks derived from the pre-Cambrian at the head of the Blue or brought down by lateral tribu- 
taries from the near-by slopes of the Tenmile Range. With these are mingled red and gray 
micaceous sandstones from the ** Weber," Maroon, and ** Wyoming" formations exposed south 
of Breckenridge; quartzite, probably in part from the **Sawatch" quartzite at the head of the 
Blue but mainly from the Dakota; and, finally, porphyries of the various kinds found in the 
district. In the excellent exposures in the Banner hydraulic workings (see PI. XV) the lower 
15 to 20 feet of the deposit, resting on the pre-Cambrian, consists of well-rounded, rather large 
cobblestones embedded in fine gravel and sand. The bowlders at this place are mostly of 
pre-Cambrian rocks, but some are quartzite. As a rule nearly all the roimded bowlders in the 
terrace gravels except those of quartzite are more or less decomposed, and those of granite, 
gneiss, or red sandstone are generally so soft that they can readily be picked to pieces. The 
disintegrated state of some of them is apparent in Plate XV, B, 

Above this bed of bowlders in the Banner placer is a 6-foot layer of stratified sand and 
clay with some scattered pebbles (PI. XV, A)^ grading upward into rather angular gravels, 
which, although consisting of very imperfectly assorted material, show rude, indistinct stratifica- 
tion. Irregularly distributed through the finer gravelly and clayey matrix of these upper beds 
are some rather angular masses of quartzite up to 5 feet in length, with smaller and more rounded 
bowlders of pre-Cambrian rocks, which are generally decomposed. 

On the opposite side of the Blue (see PI. XIII, p. 68), the terrace gravels as exposed in the 
pits between the 2-mile bridge and French Gulch contain large well-rounded bowlders of pre- 
Cambrian rocks and of red sandstone, many of which are decomposed. 

Pre-Cambrian material is abundant in the western part of the gravel mass east of Brecken- 
ridge, but in the placer pits, which are mostly on the French Creek slope of the spur, pre- 
Cambrian bowlders are rare while those of quartzite and porphyry are very common. Many of 
the porphyry bowlders, however, are so soft that they go to pieces under the impact of the 
water from the monitors or crumble later on exposure. The bowlders are rudely stratified with 
yellow sandy clay and occur mostly near the bottom of the deposit. 

The terrace gravel of the broad, low spur south of Braddocks, between Gold Run and the 
Blue, is composed of well-rounded material consisting largely of detritus from the pre-Cambrian 
terrane. On the Gold Run side of the spur this material appears to grade into the more angular 
hillside wash presently to be described. 

On the south side of the Swan, between Delaware Flats and Galena Gulch, one hydraulic 
pit exposes well-rounded terrace gravel resting on bedrock about 250 feet above the floor of 
Swan Valley. The bowlders are mostly of crystalline rock and have probably come from the 
pre-Cambrian near the heads of Muggins Gulch and of the main forks of the Swan. They are 



74 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

up to 2 feet in diameter and are generally decayed. This gravel is only about 5 feet thick at this 
place and is overlain by abopt 25 feet of hillside wash. 

The matrix in which the bowlders of the terrace gravel are held varies from coarse gravel 
to fine clay. Generally it is a sandy or clayey gravel in which are pebbles and cobblestones of 
all sizes up to those of the largest bowlders. The gravels as a rule are cemented by nothing 
harder than clay and as this is not of a particularly tough variety they can be readily broken 
down by the impact of water under moderate pressure. 

CORRELATION WITH DEPOSITS OUTSIDB OF THE DISTRICT.* 

East of the Rocky Mountains no less than six distinct advances of the ice during the 
Pleistocene have been recognized. In the middle Cordilleran region, on the other hand, the 
most careful recent study has failed to reveal more than two epochs of maximum glaciation. 
Possibly there were others, but as Atwood ^ has observed, there are peculiar difficulties in 
the way of distinguishing separate ice advances in a mountainous region owing to the readiness 
with which a late canyon glacier may obliterate the traces of an older one. 

In the Leadville district, recently studied by S. R. Capps,* the deposits left by the first 
epoch are represented by remnants of moraines and by the accumulation of terrace gravels in 
the valley of the Arkansas. Hayden ' appears to have been the first to describe these gravels. 
He regarded them as having been laid down in a lake at the close of the glacial epoch. Later, 
S. F. Emmons * also described them as "glacial or lake" beds and noted that they range from 
fine calcareous marls to coarse bowldery gravels. A fluvio-glacial origin for the terrace gravels 
near Leadville was urged by Capps and Lefllngwell * in 1904 and by Westgate • in 1905; but 
Enmions and Irving ' later stated that the terrace deposits are divisible into two parts — a lower 
one consisting of fine-grained stratified arkosic sands with some clay and an upper one of coarse 
gravels. These writers believe that the finer material is probaLly lacustrine and that only the 
overlying gravel or "wash" is of glacio-fluviatile origin. Finally, in 1909, Capps • laid stress on 
the very close relation between the terrace deposits and the older drift and maintained that the 
terrace deposits have no lacustral characteristics but were deposited by overloaded extra- 
glacial streams fed by the melting of the older glaciers. He noted that a large proportion of 
the bowlders in the terrace gravels are decomposed. 

There can be little question that the terrace gravels of the Leadville area and those in the 
valley of the Blue were laid down contemporaneously. The latter clearly represent the work 
of aggrading streams.* 

Two glacial epochs were recognized by BalP*^ in the Georgetown quadrangle and the 
remnants of older till described by him probably accumulated while the terrace gravels were 
being deposited near Breckenridge. No real morainic material belonging to the older epoch 
has been found in the Breckenridge district. 

OLDER HILLSIDE WASH. 

DEFINITION. 

Hillside wash is a rather comprehensive term that may include depoi^ts of widely different 
age and volume. Along Gold Run many of the gulches in the Breckenridge district, particu- 
larly those on the north slope of Farncomb Hill, are accumulations of soil and angular rock 

1 Atwood, W. W., Oladation of the Uinta and Wasatch Mountains: Prof. Paper U. S. Geol. Survey No. 61, 1909, p. 68. 

* Pleistocene geology of the Leadville quadrangle, Colorado:. Bull. U. S. Qeol. Survey No. 386, 1909. 
» Hayden, F. V., Kept. U. S. Geog. and Geol. Survey Terr, for 1873, 1874, pp. 52-53. 

« Geology and mining Industry of Leadville, Colo.: Mon. U. S. Geol. Siu"vey, vol. 12, 1880, p. 71. 

• Capps, S. R., and Lefllngwell, £. D. K.. Pleistocene geology of the Sawatch Range: Jour. Geology, vol. 12, 1904, pp. 702-704. 
< Westgate, L. Q., The Twin Lakes glaciated area, Colorado: Jour. Geology, vol. 13, 1895, p. 295. 

» Emmons, S. F., and Irving, J. D., The Downtown district of Leadville, Colo.: Bull. U. S. Geol. Survey No. 320, 1907, pp. 10-18. 

• Capps, S. R., Pleistocene geology of the Leadville quadrangle, Colorado: Bull. U. S. Geol. Survey No. 386, 1909, pp. 17-20. 

* Capps mapped as "drift of older epoch of glaciation" the areas west and southwest of Breckenridge that in this report are included with the 
terrace gravels. As his work did not extend far north of Breckenridge, this was a very natural conclusion, inasmuch as these particular masses of 
gravel are but poorly exposed and where they adjoin the later terminal moraine along Carter Gulch they appear to have l^en moved to some extent 
by the ice and are characterized locally by a hummocky topography. Mr. Capps in conversation has agreed to the change in designation here noted. 

>• Ball, S. IL, Prof. Paper U. S. Geol. Survey No. 63, 1908, pp. 84-85. 




.4, GENERAL VIEW OF THE GOLD RUN PLACERS FROM THE SOUTHEAST. 
TobaOly the most productive and extensively worked hydraulic washings In the district, are m the older hM\ 
wash. Sea page 1 77. 




B, NEAR VIEW OF A BANK IN THE GOLD RUN PLACERS. 

ie angular form of the fragments. The bank is about 40 feet high. See pat 



QUATERNARY DEPOSITS. 76 

detritus that have been derived from the immediately adjacent slopes by creep and by the 
wash of rain water or snow water in its descent to the established stream channels. Many of 
these deposits have been rich in detrital gold and yielded lai^e returns to the early placer 
miners. Some are not definitely distinguishable as r^ards either age or thickness from the 
soil that mantles the hills to various depths; some, on the other hand, are of notable bulk and 
are so closely related to the terrace gravels as to suggest reference to the same general period 
of deposition. The distinction between the two is one of age rather than kind, and probably 
would be even less definite were the history of the deposits fully known. In other words, as 
the erosional development of the region proceeded, old detrital deposits were reworked into new 
and probably passed through intermediate stages whose obscure traces are very illegibly recorded 
in the topography of to-day, and consequently when found are likely to be assigned to the 
banning or end of a cycle rather than to the intermediate place where they properly belong. 
It is accordingly recognized that the distinction here made between the older and younger 
hillside wash is perhaps unnaturally rigid. The deposits mapped as older wash (PI. I, in 
pocket) are those having an extent, thickness, and range of altitude that are out of proportion 
to the present activity of the neighboring streams and that relate them to the physiographic 
and climatic conditions similar to those under which the terrace gravels were laid down. Other 
deposits, generally smaller, plainly related to existing ravines and distinguished from ordinary 
local thick accumulations of stony soil merely by their auriferous character, are r^arded as 
younger or recent hillside wash and are given no distinctive color on the map. 

DISTBIBTJTION. 

The principal areas of older hillside wash are along the southwest a^de of Gold Run and one 
on the south side of the Swan, just east of Delaware Flats. Both, but particularly the Gold 
Run deposit, have been extensively worked by the use of monitors or giants. 

THICKNlfiSS. 

In Gold Run the auriferous wash is up to 40 feet thick. Most of it rests on shale, although 
at the upper or southeast end of the deposit the bedrock is partly porphyry. There is no dis- 
tinct rock terrace, but the detrital material Ues on a gentle slope and merges along its upper 
margin with the ordinary soil of the hillsides. The upper limit of the deposit, so far as it can 
be said to have a limit, is from 250 to 300 feet above the present bedrock channel of the stream 
draining Gold Run. 

The wash of the area on the south side of the Swan rests upon an uneven surface of por- 
phyry and shale and overlies, as has been mentioned, at least one thin deposit of terrace gravel 
that rests on a high rock bench. The older wash of this area attains a maximum thickness of 
25 to 30 feet. 

CHABACTEB OF MATERIAL. 

The distinguishing features of the hillside wash as compared with the terrace gravels are 
the angularity of its fragments and the local derivation of these. This angular character is 
well shown in Plate XVI, which illustrates the appearance not only of the undisturbed material 
but of the large fragments left after the finer material has been sluiced away. In this view the 
fragments are chiefly Dakota quartzite from the north spur of Gibson Hill. Some are as much 
as 3 feet long, but most are less than 1 foot in greatest diameter. In the bank these fragments 
are mingled with much soft yellow sandy clay and small angular rock particles, the whole 
showing rude stratification with a gentle dip corresponding in general with the slope of the 
bedrock. In comparison with the terrace gravels, the proportion of large rock fragments 
to the earthy matrix is small. The very large proportion of durable quartzite fragments in 
the hillside wash renders it difficult to compare the effect of weathering processes on this mate- 
rial with the conspicuous results noted in the terrace gravels. Where fragments of porphyry 
occur in the wash, however, they are generally much less decomposed than the bowlders of 
similar rock in the terrace gravels. 



76 GEOLOGY AND ORE DEPOSITS OF BEECKENRIDGE DISTRICT, COLORADO. 

The terrace gravels are distinctively fluviatile — the deposits of powerful and persistent 
streams capable of moving large bowlders for long distances. The hillside wash, on the other 
hand; represents the action of shifting and temporary floods cooperating with alternating 
freezing and thawing, or wetting and drying, with disturbance of local equiUbrium by the 
growth or decay of vegetation or by the disintegration of the rocks and with all the small but 
inevitable forces that tend to move loose material downhill under the stress of gravity. 

RELATIVE AGE OF THE TERRACE GRAVELS AND OLDER HILLSIDE WASH. 

Near the mouth of Gold Run the terrace gravels and the older hillside wash come together 
in such a way as to suggest contemporaneous deposition. Unfortimately, however, there are 
in this vicinity no good exposures of the two kinds of material, so that it is impossible to say 
whether the suggestion of gradation of one into the other has any real basis or whether the wash 
is wholly yoimger than the gravel and was deposited against it. In the area east of Delaware 
Flats some well-rounded coarse gravel, supposed to belong to the same period as the terrace 
gravel along the Blue, occurs at the base of the hillside wash and is accordingly older than the 
wash at this particular place. This fact, in connection with the comparatively undecomposed 
condition of the wash, indicates that it is probably all somewhat younger than the terrace 
gravels. How much yoimger can not be definitely determined, for it does not appear possible 
to relate the hillside wash in any definite way with the morainal deposits of the second ice 
advance or with any other saUent physiographic event, 

MORAINES. 

DISTRIBUTION. 

The morainal deposits of the Breckenridge district belong, so far as can be determined, 
entirely to the second stage of glaciation. They attain their greatest volume on the Blue and* 
its tributaries south and west of the town of Breckenridge. Smaller masses of similar material 
are to be seen on French Creek, both near Lincoln and a mile farther upstream south of Fam- 
comb Hill. Excellent examples of terminal and lateral moraines may be studied also on the 
forks of the Swan, but the ice-borne material in that valley barely entered the area mapped in 
coimection with this report. 

MORAINES ON BLT7E RIVER. 

The ice of the last advance filled the valley of the Blue from the cirques at its head to the 
site of Breckenridge, a distance of about 10 miles, and the terminal moraine left by this glacier 
just south of the town is a notable feature of the valley topography. The farthest north reached 
by the ice appears to be naarked by a morainal remnant in the form of a low ridge of bowlder 
till that extends northwest from the base of Nigger Hill to the vicinity of the Gold Pan shops. 
'The sharp morainal ridge between Carter Gulch and the Blue was perhaps formed or initiated 
at this time of maximum ice extension. 

The glacier then retreated slightly and began the formation of the main terminal moraine, 
whose front stretches westward across the valley from the base of Little Mountain. The main 
road south from Breckenridge traverses a smooth plain of low-level gravel for about a third 
of a mile and then makes a short ascent of about 50 feet to the top of the moraine, which shows 
a typical knob and kettle topography. From the road as it skirts the south edge of this lote to 
descend into the recent trench that the river has cut through the moraine, or, better still, from 
Little Mountain, an excellent view may be obtained over the hummocky surface of the terminal 
moraine, which extends for fully 2 miles south of Breckenridge. (See PL XVII, A,) Beyond 
it is the broad upper valley of the Blue, once occupied by ice but now floored with gravels 
and silts deposited in front of the glacier as it retreated southward. The features of this valley 
that are recognized as due to glaciation have been described by Capps,^ who calls attention 

» Capps, S. R., Bull. U. 8. Geol. Survey No. 386, 1909, pp. 77-79. 



J. S. ttEOUOaiCAL SUftVEV 



PBOFESSIONAL P, 




A. UPPER VALLEV OF THE BLUE, WITH TERMINAL MORAINE IN THE FORE AND MIDDLE 
GROUNDS. 

a view is south, from Little Mountain. In the foreground is the present channel of the river through the moiain* 
Back of the moraine is the little area of level silt knov^n as the Goose Pasture. Hoosier Pass is visible in th 
disUnce. See page 76. 




QUATERNARY DEPOSITS. 77 

particularly to the fine examples of glacial sculpture displayed at the head of the Blue (Monte 
Cristo Gulch) and in the cirque north of Quandary Peak. He does not mention, however, the 
occurrence of two well-marked though low moraines of recession, each with its gently sloping 
train or outwash apron, which is now, of course, dissected by the present river into two lateral 
terraces. The road up the valley lies for much of the way on these terraces, ascending each to 
its head, crossing a low moraine, and dropping to the foot of the next gradual slope. 

Indiana Creek finds its way into the Blue through a narrow cleft which it has cut through 
a mass of morainal material that once blocked its course. There is no evidence, however, that 
any glacier came down Indiana Gulch. The material was probably dropped as a lateral moraine 
and crowded into the mouth of the gulch by the main glacier that formerly occupied the valUey 
of the Blue. 

The material of the terminal moraine consists of the unassorted angular material charac- 
teristic of such deposits. The rock fragments are chiefly of pre-Cambrian crystalline rocks 
from the Tenmile Range, but there is also much quartzite. At one place on the extreme south 
edge of the area mapped, near the head of Carter Gulch, the blocks of quartzite are so numerous 
as to indicate a probable outcrop of tliis rock through the moraine. The largest mass of trans- 
ported rock noted in the terminal moraine is a fragment of gneiss east of the so-called Goose 
Pasture. This measured 30 feet in length, 15 feet in width, and at least 12 feet in height, a part 
of it being buried. 

The moraines west of Breckenridge were formed by short lateral glaciers that headed in 
cirques near the crest of the Tenmile Range and deposited debris from the pre-Cambrian 
crystalline rocks over the terrace gravels. 

MOBAINES IN FBBNCH QJJUCIL 

The French Creek glacier advanced fully half a mile below Lincoln and left morainal 
(^eposits on both sides of the valley. That of Weber Gulch, on the south, is the more extensive 
^nd attains an altitude of 9,750 feet, or 550 feet above French Creek. The moraine north of 
tincoln (see PL I in pocket) rises to about 9,650 feet. Probably these two masses were at one 
time connected by a terminal moraine across the valley, but if so, this appears to have been cut 
through before the end of the glacial epoch. The remaining material evidently was dumped 
and crowded by the ice into the two Uttle ravines that happened to open opposite each other 
near the end of the glacier and probably includes also considerable detritus contributed by the 
ravines themselves while their mouths were blocked by the ice. As the glacier retreated up 
the valley whatever terminal moraine may then have existed was cut through and part of the 
morainal masses on the hillsides, no longer supported by the ice, slid down over the slopes against 
which the glacier had rested and mingled with the low-level gravels. 

The material of the Weber Gulch moraine is well exposed to a depth of about 50 feet in some 
old hydrauUc workings, but the base of the deposit is not now visible. The banks show an 
unstratified mixture of large and small, round and angular fragments up to 6 feet in diameter, 
with much earth, sand, and clay. Most of the larger blocks are quartzite or porphyry, such 
as might have been derived from the head of the gulch. There are many smaller fragments 
of shale, however, and some of these show well-preserved glacial stride. A few of the porphyry 
blocks also show striae, but tliis rock, owing to its coarser texture, is not so well fitted as some 
of the harder varieties of the shale to receive and retain glacial scratches. The material in Rich 
Gulch, north of Lincoln, is similar to that of Weber Gulch, but few of the fragments here exceed 
4 feet in diameter and most are under 18 inches. Striated fragments of shale are abundant. 
The Uttle meadow known as Lincoln Park (see PI. V, -4, p. 20) is due to the silting up of a short 
ravine behind the moraine. There was possibly a lakelet here in Pleistocene time. 

The next recorded halt of the retreating end of the French Creek glacier was in the vicinity 
of Black and Little French gulches, south of Famcomb Hill. Here was left a terminal moraine 
that still retains its characteristic hummocky topography. The upper Umit of morainal deposi- 
tion is here, as elsewhere in the district, rather indefinite, but some material was certainly dropped 
on the hillsides at least 500 feet above the bottom of the valley. At the mouth of Little French 



78 GEOLOGY AND OEE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

Gulch is a well-preserved morainal bench about 400 feet above French Creek with an undrained 
depression on its top. This material, which is largely shale with abundant striated and faceted 
fragments, was probably dumped into Little French Gulch from the main glacier, forming a 
dam through which the small present stream has cut a narrow gorge, apparently a little south 
of its preglacial channel. 

There is much rocky detritus in the basin at the head of French Creek, but this is not defi- 
nitely morainal and in large part is certainly debris that has fallen in postglacial time from the 
steep rocky walls against which the ice formerly exerted its supporting pressure. 

Naturally, there are no fragments of pre-Cambrian rocks in any of the French Gulch 
moraines. Their constituent fragments are porphyry and shale with a subordinate quantity 
of quartzite and limestone, all derivable from the area now drained by the creek. 

MORAINES ON THE SWAN. 

The morainal material shown along the Swan opposite Georgia and American gulches 
appears to represent the farthest advance of the ice down that river valley. The moraine 
mapped consists of the usual coarse till with huge blocks of pre-Cambrian rock and is merely 
the outer edge of a terminal moraine deposited by glaciers that came down the south and middle 
forks of the Swan. There is a succession of hummocky terminal moraines with numerous 
kettles for at least a mile up the South Swan and then ground moraine up to Georgia Pass, which 
appears to have been itself covered with ice or n6v6. The Middle Swan also affords an excellent 
example of a typical and well-developed terminal moraine with some lateral moraines farther 
upstream. Scattered over the terminal moraine and on adjacent slopes are some large erratics 
of pre-Cambrian rocks, some being over 25 feet long. The distribution of these bowlders shows 
that the Middle Swan Glacier can hardly have been less than 400 feet thick near its distal end. At 
one time during the last ice epoch the drainage from the glacier appears to have flowed through 
a col on the north side of the valley near its lower end and joined the main Swan below the 
present mouth of the Middle Swan. 

Some scattered erratics of pre-Cambrian rock lie on the shale slope north of the mouth of 
Georgia Gulch up to a height of 400 feet above the Swan. 

» 

LOW-LEVEL GRAVELS. 

OCCTJBBrENCE. 

The low-level gravels, on which depends the present dredging industry of the district, occupy, 
as their name implies, the bottoms of the existing valleys. Their greatest known thickness, 
about 90 feet, is near the Gold Pan pit between Breckenridge and the terminal moraine. Along 
French Creek the depth to bedrock in the main channel, from Nigger Gulch down, ranges from 
45 to 50 feet. Along the Blue, between Braddocks and the mouth of the Swan, the depth in 
the old channel is from 55 to 60 feet, and along the Swan, from Galena Gulch down, the max- 
imum thickness is from 40 to 50 feet. The Blue north of the moraine, French Creek below 
Lincoln, and the Swan below Georgia Gulch are thus all flowing in channels aggraded many 
feet above the original rocky beds of these streams. This aggradation dates from the later 
glacial stage and the low-level gravels are valley trains, deposited by heavily overladen streams 
fed by the melting glaciers. 

Some of the present streams have cut trenches in the low-level gravels so that their narrow 
alluvium-covered flood plains are distinctly lower than the even surface of the glacial outwash 
gravels. Near the Gold Pan pit, south of Breckenridge, the bluffs of the low-level gravel rise 
from 15 to 20 feet above the strip of alluvium bordering the river; about 2 miles below Breck- 
enridge the difference in level is not over 10 feet; opposite Braddocks it is reduced to about 
5 feet; and at the mouth of the Swan there is no sharp distinction between the two surfaces. 
The Swan, from a point a mile above Swan City to its mouth, appears to have spread its modern 
alluvium over the low-level gravels without trenching. Along French Creek, on the other 
hand, particularly in the vicinity of Lincoln, the bluffs of low-level gravel rise from 15 to 20 
feet above the stream. 



PROFESSIONAL PAPER 71 




BANK OF THE GOLD PAN PIT, SHOWING CHARACTER OF DEEP GRAVELS ALONG THE BLUE, 
e pit is nearly 90 feet deep, and probably not more than 10 to 30 feet of the gravel is here eiposed. Note iar^e siza 
of bowlders and lack of stratification. See page 17B. 




B. BOWLDERS FROM THE GOLD PAN PIT. 
Some of these are 6 feet long. See page 1 78, 



QUATERNARY DEPOSITS. 79 

A good idea of the generally even surface of the low-level gravels may be had from Plate 
XrV, A (p. 70), which is a view up the valley of the Blue from a point opposite the mouth of 
French Creek. The view shown in Plate XIII, A (p. 68) is one seen in looking north-northeast 
from the same place. In the immediate foreground is the entrenched flood-plain of the Blue, 
with its characteristic growth of willows; beyond this is a terrace of low-level gravel corre- 
sponding to that on which the observer stands, and rising 40 to 50 feet above this is a bench 
of the older terrace gravels. A similar relation of alluvium, low-level gravels, an.d terrace 
gravels is shown in Plate XIV, B (p. 70), which is a view west from Gibson Hill, across the 
valley of the Blue. 

The low-level gravels attain their maximum width, about 3,000 feet, near Breckenridge 
and Braddocks. As will be more fully shown in the chapter on placers, they were deposited 
in wide-bottomed channels, unconfined by steep banks and of low gradient. 

MATERIALS. 

The character and state of aggregation of the material composing the low-level gravels are 
such important factors in dredging operations that their detailed consideration may well be 
deferred to the chapter on placers. It is siffficient to note here that the gravels are unce- 
mented and are generally coarse, with hard well-rounded bowlders. In the gravels along the 
Blue the diameters of these bowlders range from a maximum of about 6 feet near Brecken- 
ridge to 4 feet near Valdoro. In French Gulch the large bowlders rarely exceed 3 feet in greatest 
diameter and are not so well rounded as those on the Blue, many of which were shaped prior 
to the second ice epoch. On the Swan, below Galena Gulch, the gravel is more uniform and 
contains fewer large bowlders than that along the Blue or in French Gulch. Coarser material, 
however, is found above the mouth of the North Fork of the Swan. 

The low-level gravel shows no recognizable stratification and the largest bowlders are not 
limited to the bottom of the deposit, but may occur at any horizon. This nonassorted mate- 
rial is well shown in Plate XVIII, A^ which is a view of the upper part of the gravel at the Gold 
Pan elevator pit. There are no places at present where the gravels can be seen below water 
level, but it is probable that were a series of complete sections of the gravel available for 
examination those farther from the moraine than the Gold Pan pit would show less chaotic 
deposition. 

GLACIAL. LAKE BEDS. 

The little area of meadowland known as the Goose Pasture, partly inclosed by the 
terminal moraines south of Breckenridge, probably corresponds to the depression occupied 
by the attenuated end of the waning glacier, before it retreated definitely southw^ard near the 
close of the second ice epoch. Apparently the Blue had at this stage already established its 
channel across the older part of the terminal moraine, so that the final retreat of the ice left 
only a small lake to occupy its abandoned bed behind the moraine. This presumably was soon 
filled. Of the character of this deposit very Uttle could be learned. According to Mr. B. S. 
Revett a drill hole put down in the Goose Pasture passed through 106 feet of silt, sand, and 
fine gravel, but detailed records of this boring could not be found. Superficial natural expos- 
ures in the banks of Blue River show sand and gravel, the pebbles rarely exceeding 4 or 5 
inches in diameter. Bedding is not apparent. 

RECENT ALLUVIAL AND DETRITAL DEPOSITS. 

Under Recent alluvium are included the thin deposits of unconsolidated material laid 
down on their flood plains by the streams and streamlets of the present cycle. Here belong 
the meadowlands along the Swan and unimportant strips of lowland, generally clothed with 
willow thickets, along French Creek and the Blue. Lincoln Park is covered with alluvial soil 
washed from the surrounding slopes and possibly resting on older lake silts. 

Here may be mentioned also the loose material, not as a rule separable from the ordinary 
soil of the hillsides, that in early days was washed for gold in American, Georgia, Humbug, 



80 GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

Browns, Galena, Gibson, Nigger, and other gulches. This will be referred to again in the chapter 
on placers. ' 

The residual soils are in many places so deep and so rich in vegetable matter as to afford 
to the geologist or prospector no reliable indication of the kind of rock beneath them, especially 
as in this region of former glaciation and intricate intrusion loose rock fragments are very unre- 
liable guides to underiying structure. On such slopes the shale is likely to be particularly 
elusive, as was found to be the case on the lower northeast slope of Bald Mountain and on the 
northwest slopes of Brewery and Humbug hills. The gentle slope of Humbug Hill in the 
vicinity of the northwest comer of sec. 35, T. 6 S., R. 77 W., is covered with soil containing 
abundant fragments of quartz monzonite porphyry and seemingly derived in place from that 
rock. Numerous pits, however, penetrate loose porphyry detritus, in places 6 to 7 feet thick, 
and expose shale beneath. Presumably the shale at this locality was formerly covered by a 
sheet of porphyry, perhaps continuous with that now capping tho crest of the hill. Erosion 
has cut through this porphyry to the shale but has not yet been able to remove the residual 
fragments of the harder rock. 

There are considerable accumulations of angular rock waste at the head of French Gulch 
and in similar cirqueUke basins outside of the mapped area. For the most part this material 
is ordinary postglacial talus, although it is possible that some of it was deposited during the last 
stages of ice occupancy. None of the material seen exhibits the remarkable features of the rock 
streams described by Howe in the San Juan Mountains.^ 

» Howe, Ernest, Landslides In the San Ju&n Mountains, Colondo: Prof. Paper U. 8. Oeol. Survey No. 67, 1909. 



J. S. GEOLOGICAL SURVEV 



PROFESSIONAL PAPER 7S PLATE X 




ze and brmgs Out clearly the cha-a- 
faces of the octahedron Phatcir 
n of Natural History, Donvor. s4' 



CHAPTER VI. 
MINERALOGY. 

INTRODUCTORY STATEMENT. 

Mineralogically the Breckenridge district, although a locality noted for the occurrence of 
crystalline native gold in filiform and leafy aggregates of remarkable delicacy and beauty, is 
not of great or varied interest. The ores in general offer little attraction to the mineralogist, 
and the common minerals constituting them call for no extended description. The order in 
which these are treated is that followed in Dana's *' System of mineralogy." 

NATIVE ELEMENTS. 
STJLFHUB. 

* Native sulphur is known only from the Iron Mask mine, where, intimately mixed with 
earthy cerusite and probably with some anglesite, it is said to have been found on the bottoms 
of cavities in the oxidized ore. Native sulphur probably often forms or is set free in the 
oxidation of sulphide ores, but, as a rule, is quickly oxidized to sulphiuic acid. In this case 
the oxidation was incomplete. 

GOLD. 
CRYSTALLIZED GOLD OP FARNCOMB HILL. 

Native gold occurs in the Breckenridge district both in veins and placers. The principal 
locality for vein gold is Famcomb Hill, whence have come the innumerable specimens of "Breck- 
enridge gold" found in museums and private collections the world over. It is probable that 
from no locality equally productive has so lai^e a proportion of the gold mined been saved from 
the melting pot on account of the beauty and interest of its form. 

The typical gold from the narrow veins of Famcomb Hill occurs aa two general varieties 
locally known aa wire gold and leaf or flake gold. It is stated by those famiUar with the tradi- 
tions of the hill that wire gold is characteristic of the veins west of American Gulch and leaf gold 
of those east of that ravine. 

The appearance of typical flake specimens is well shown in Plates XIX to XXI, representing 
specimens given by Mr. John F. Campion to the Colorado Museum of Natural History, Denver. 
The photographs from which these illustrations were made were obtained through the courtesy 
of Dr. W. S. Ward, curator of the museum. The largest specimen in this notable collection is 
about 8 inches long, 6 inches wide, and from one-fourth to three-fourths of an inch thick. The 
heaviest single mass ever obtained from the hill came from a tunnel on the Gold Flake vein 
and was found by H. J. Litten and others about the year 1887. It was jocularly named "Tom's 
baby" by the miners, and is reported to have weighed 13 pounds. As shown in the illustrations, 
the leaf gold is in exceedingly irregular masses, many of which consist of thin septa meeting at 
angles that strongly suggest deposition controlled in part by the cleavage planes of a rhombo- 
hedral carbonate of the calcite group. As a rule the sides of these septa and other less regular 
surfaces of the mass are covered with small crystal faces, generally the triangular face of the 
octahedron, up to about 3 millimeters in diameter. In some places these faces are irregularly 
crowded, distorted, and associated with planes not so clearly belonging to the octahedron. In 
other places the little triangular facets are in parallel orientation and suggest a regular imbrica- 
tion of golden scales. Skeletal crystals are common, especially octahedrons, in which each face 
has a triangular depression or panel. 

90047**— No. 75—11 6 81 



82 GEOLOGY AND ORE DEPOSITS OP BBECKENBIDGE DISTRICT, COLORADO. 

The wire gold, of which no good illustration has been obtained, is fully as beautiful as the 
leaf gold, but being more fragile it is not often secured in as large specimens. It consists gen- 
erally of a porous mass of curved and intricately felted or tangled crystalline wires, which may 
individually be 3 inches or more in length. There are all gradations between coherent spongy 
masses of felted wires and single filaments, but as a rule, where the gold occurs in commercial 
quantity, it is aggregated into masses of considerable size. The wires vary greatly in thickness 
and shape. Some are short, stout, and rodlike; others are of hairlike slendemess. Some are 
longitudinally striated and resemble curved capillary shavings such as might be made with a 
graving tool ; others are transversely ridged and are evidently chains of crystals regularly grown 
together, or may be regarded as one tremendously elongated crystal exhibiting the phenomenon 
known as oscillatory growth and illustrated familiarly by the striated prisms of slender quartz 
crystals; still others are rough or frosted in appearance and as seen under the microscope are 
covered with tiny crystal facets much as are the surfaces of the leaf gold. In cross section the 
wires may be square, ribbon-like, or irregular, and they may vary in thickness from point to 
point. Some are simple; others branch and send off little curled tendrils. In fact no wire is 
straight, and most of them are bent and twisted in various directions. Specimens of the wire 
gold, while all possessing a character distinctive of Famcomb Hill, are never monotonous. 
Each has its individuality of texture, crystallization, aggregation, shape, and shade of color. 
Notwithstanding the extreme tenuity of many of the golden filaments, these are generally so 
interwoven as to give the mass a coherency greater than would at first appear to belong to so 
delicately beautiful a mineral aggregate. 

The thickness of the masses of leaf or wire gold is generally less than an inch, and, as a rule, 
nearly or quite equals the thickness of the vein in which they are found. The gold, however, 
does not occur in the clean, bright condition familiar in cabinet specimens, but is normally 
embedded in a reddish earthy matrix consisting largely of limonite with oxides and carbonates 
of copper and various earthy impurities. The removal of this matrix by baths of acid, par- 
ticularly of hydrofluoric acid, is a process requiring exceptional patience and skill — qualities 
which, when *^gold strikes" were more frequent than now, brought their possessor into much 
demand. So far as could be learned all the important masses of gold taken from the veins have 
been associated with this oxidized matrix. The association of gold and galena, however, is not 
unusual. Some specimens show crystals of galena implanted on or inclosing threads of gold. 
Some of the deepest gold found in Famcomb Hill, from stopes above the Fair tunnel, is asso- 
ciated with sphalerite in calcite. Wire gold in sphalerite is reported also from the I. X. L. mine, 
on the Swan near Browns Gulch, and some of the nuggets dredged from the gravels of French 
Creek show a similar association. Wire gold embedded in calcite has been obtained from the 
Key West vein at a depth of about 250 feet. 

The vein gold of Famcomb Hill varies from about 750 to 900 fine, bringing from $15 to 
$18.50 an ounce, or more for exceptional specimens. 

OTHER OCCURRENCES OF GOLD IN VEINS. 

The occurrence of native gold in veins is not limited to Farncomb Hill, although no other 
locality in the district has produced masses equal to those of the hill in size or in beauty of form. 
The ore from the Jumbo and Extension mines, on the north slope of Gibson Hill, contained much 
free gold in the upper oxidized parts of the veins. Some wire gold associated with native silver 
has been found in the Juventa mine, northeast of Lincoln. Specimens of pale wire gold in 
galena, seen in Breckenridge collections, are said to have come from rich pockets in the generally 
low-grade ore of the Cashier mine in Browns Gulch, and a pale low-grade gold is reported to 
have been obtained from the now abandoned Helen workings, on the south side of French Gulch. 
Bunches of oxidized ore containing free gold have been found in the fractured Dakota quartzite 
at several places, particularly on Shock Hill and Little Mountain. In the latter hill was uncov- 
ered on0 bunch that produced $30,000 from rich Umonitic ore containing native gold and silver. 
Handsome specimens of native gold have been taken from some of the mines in the pre-Cambrian 
rocks at the head of the Blue, especially from the Senator, Arctic, and Ling mines. A specimen 





NATIVE GOLD FROM FARNCOMB HILL. 



ral sue. The upper shows the usual Intersecting 
inted octahedral crystals, partly m parallel onentati 
:lion, Colorado Museum of Natural Hiitoty, Denve 



lates. The lowei illustrates partici 
1. Photograph by Schwartz from a 
See page Bi. 



MINERALOGY. 83 

from the Arctic mine seen in 1909 showed the gold intergrown with bismuthinite and associated 
with pyrite and chalcopyrite in a quartz gangue. This gold has none of the crystalline structure 
characteristic of so much of the metal found near Breckenridge. 

The localities mentioned are by no means the only ones at which native gold in place has 
been found in the Breckenridge district, but they include the more important occurrences. 
While a highly crystalline structure is especially characteristic of the gold from Famcomb Hill, 
the tendency to crystallinity and to the assumption of the wire and leaf forms is a noticeable 
feature of nearly all the gold mined in the area here mapped. 

PLACER GOLD. 

• 

The placer gold of the Breckenridge district has considerable resemblance to the lode gold 
and much of it, in spite of the wear to which it has been subjected in its travels, shows crystalline 
structure. Many of the nuggets are clearly more or less battered fragments of leaf or wire gold, 
although, as would naturally be expected, the nuggets in general are more solid than the speci- 
mens of wire gold from the Famcomb Hill veins, as only those pieces having initial coherence and 
solidity could survive the pounding to which they are subjected by the stream bowlders. 

Of the character of the gold formerly washed from the Famcomb Hill placers Uttle can 
now bo learned at first hand, but the gold appears to have been only slightly worn. Even on the 
Swan at Snyder's camp, where the gravels are 27 feet deep and contain many huge bowlders, 
much of the placer gold retains its leafy or wiry character, although some of the nuggets are 
well rounded. Among the nuggets seen from these workings on the Mascot placer were some 
of the general size and shape of beans, and some flat scales a millimeter or so thick, 5 millimeters 
wide, and 15 milUmeters long. Lower down the Swan the proportion of rounded nuggets 
increases and gold recognizable as of Famcomb Hill derivation becomes less abundant. The 
lai^est nugget found is said to have weighed 29i ounces Troy, and was dredged from the mouth 
of Galena Gulch. Such large masses, however, are wholly exceptional on the lower Swan, and 
the dredges now at work would probably lose over the stacker any nugget exceeding an inch in 
diameter. The placer gold of the lower Swan is about 770 fine. 

In the deep placers of French Gulch, where dredging is now in process, the gold is coarse 
and nuggets 3 ounces in weight are fairly common. Many show wiry or crystalline structure. 
Some have adhering particles of quartz or contain gold embedded in sphalerite. They vary 
much in shape, but a large proportion are more or less flat or platy, the thickness of many 
corresponding to the width of the narrow veins in which they once lay. The fineness of the 
French Creek gold ranges in general from 750 to 760. 

SILVER. 

Native silver is not at present of common occurrence in the Breckenridge mines, and none 
was seen in 1909. It is said, however, to have been found in the Liberty mine, now a part of 
the Wellington. According to Mr. Peter Cuxninings, superintendent of the Wellington, tliis 
silver formed aggregates of coarse wires in a soft part of the vein. Native silver is reported 
also from the Juventa mine, near Lincoln, and from the placers formerly worked at the west 
base of Little Mountain. Although now rare, native silver probably occurred in small quan- 
tities in many of the oxidized parts of the galena-bearing lodes formerly worked. 

SULPHIDES. 

BISMUTHINITE. 

A lead-gray mineral, called a telluride ^ by the miners, but determined by W. T. Schaller 
as bismuthinite, occurs in the Arctic vein at the headwaters of the Blue, associated with pyrite 
and native gold in a quartz gangue. The mineral is not in distinct crystals. The sulphide of 

I No tellurides were seen In the course of the investigation of the Breckenridge district In 1909. It may be recalled, however, that Dr. R. Pearoe 
(Trans. Am. Inst. Mln. Eng., vol. 18, 1890, p. 451) found tellurium in some of the LeadvlUeore, and tellurides are occasionally reported by proe- 
pectors from different places along the Tenmile Range. These latter occurrences may, however, be bismuthinite. 



84 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

bismuth occurs also in the I. X. L. mine with a very compact pale sulphide of iron, probably 
marcasite, and some pyrite, in quartz. A bismuth xnineral, probably bismuthinite, has been 
reported also from the Wire Patch and Cashier mines. 

OALBNA. 

The sulphide of lead is one of the commonest and most important sulphides found in the 
district, being exceeded in abundance only by pyrite and sphalerite. In many ores, such as 
those of the Wellington mine, it is the most valuable mineral constituent, and even in deposits 
Uke that of the Jessie mine, where gold and silver constitute the essential part of the output, 
the presence of galena is one of the best indications of good ore. Even the gold veins of 
Famcomb Hill and the high-grade zinc ore of the Country Boy mine are not free from this 
sulphide. 

The relation between the presence of galena and the silver contents of the ores is close 
and the mineral is generally though varyingly argentiferous. 

The usual form of occurrence is in moderately coarse massive aggregates. Some of the 
lai^est cleavage faces seen were in the Wellington mine and were up to 2 inches acroso. In that 
mine cavities in the ore lined with crystals of galena are abundant, generally with well-developed 
faces of the cube, modified by facets of the octahedron. 



Zinc blende is an abundant and important constituent of most of the Breckenridge ores 
and occurs also to some extent in disseminated condition in the altered porphyries and in 
arkosic sediments, close to some of the ore bodies. Most of it is a nearly black variety with dark 
reddish brown streak, but a second and economically unimportant generation of a rosin- 
colored sphalerite is found in a few ores. Well-formed crystals of the dark variety are rare. 
The habit of the mineral is generally that of a massive aggregate in which are little vugs lined 
with crystal faces. 

Sphalerite, neariy free from other sulphides, occurs in the Country Boy mine, which has 
shipped many cars of ore ranging from 47 to 55 per cent of zinc. Pure sphalerite (ZnS) contains 
67 per cent of zinc. As most dark sphalerites contain from 10 to 15 per cent of iron, the best 
Country Boy ore evidently is nearly free from gangue and other sulphides. Sphalerite is very 
abundant also in the ore of the Wellington mine, in many places preponderating over the pyrite 
and galena, with which it is generally associated. 

The frequent association of sphalerite with gold has been described in connection with that 
metal and probably none of the unoxidized ores of the district are entirely free from zinc blende. 
In the upper parts of some of the veins it alters partly to smithsonite and in the general oxidation 
of the ore deposits is decomposed and carried away in solution more readily than the associated 
galena. 

CHALGOPYBITE. 

Chalcopyrite is of rather wide distribution in the Breckenridge region, but, so far as is 
known, is not present anywhere in large quantity and is not an important ore constituent. 
It occurs in small irregular bunches in the pre-Cambrian schists of the northeast corner of the 
mapped area, but prospecting has failed to expose any bodies of commercial importance. It 
is coxnmon in the little veins of Farncomb Hill, where, as a rule, it is partly oxidized to limonite 
and malachite, and is rather sparingly present in the Wire Patch ore with pyrite, sphalerite, and 
galena. Careful search would probably reveal a Uttle chalcopyrite in most of the sulphide 
ores of the district, but many specimens show no trace of this mineral. It is generally 
associated in small quantity with the metamorphic effects described on pages 93-94. It is 
a constituent also of the ores of the Dunkin workings on Nigger Hill, the I. X. L. mine on 
the Swan, and the Monte Cristo, Senator, Arctic, and other mines in the pre-Cambrian and 
Paleozoic rocks at the headwaters of the Blue. 



J. s. aEou>GiCAL eunvEV 




NATIVE GOLD FROM FARNCOMB HILL. 
Both illustrate on natural scale the S'le and delicacy attained by soma of the cr,istal-( 
shows octahedral crystals in parallel onenlation. Photograph by SchwartJ from a spi 
Colorado Museum ol Natural History, Denver, Sea page 8>. 



HINEBAIX»T. 85 

PTBITB. 

The common disulphide of iron is the most ubiquitous sulphide in the district. It l9 a con- 
stituent of all the unoxidized ores and to a variable extent of nearly all the rocks in the region. 
In some ore deposits it is abundant, and, except where notably auriferous, generally lowers 
materially the value of the ore. At the Wellington mine the pyrite ia for the moat part a waste 
product, although in some of the middlings from the jigs, sold on their zinc contents, the pyrite 
is utilized for acid making. Pyrite unaccompanied by sphalerite or galena is rarely auriferous 
enough in this district to rank as ore. 

Part, at least, of the pyrite in the wall rock of veins, and probably all of that distributed 
through the propylitically altered porphyries has been formed without the introduction of 
additional iron, by the decomposition of magnetite and femic siUcates originally present in the 
rock, 

Mineralogically the pyrite presents no features of exceptional interest. In the form of 
sharp simple cubes, up to about 5 millimeters across, it is abundant in the ore of the Sultana 
mine, at the west base of Gibson Hill, where it is associated with chlorite, magnetite, apecularite, 
quartz, and calcite. Smaller, less r^ular cubes were noted in a vein cut by the French Creek 
tunnel, associated with much massive pyrite and some sphalerite and siderite. In the Sallie 
Barber mine much of the pyrite is in variously modified cubic crystals. Small irregular cubes 
occur also in the Wellington ore. Most of the pyrite in the district, however, is pyritohedral or 
in massive aggregates destitute of identifiable crystal faces. Some of the lai^^t pyritohedrons 
observed are in the Wire Patch 
ore, several of them being IS 
millimeters in diameter. 

An interesting case of re- 
placement by pyrite was seen 
in black calcareous shale cut by 
a lessee's tunnel on the Dunkin 
ground, on the north side of 
Illinois Gulch. Here the pyrite, 
partly in small cubic crystals 
and partly in compact abro- 
gates with a suggestion of radial structure, forms small lenses parallel with the bedding of the 
shale. Some of these lenses end sharply at minute fissures, many of them nearly microscopic, 
which themselves contain a little pyrite and calcite, or other carbonate. The specimens (see 
fig. 9) illustrate neatly on a small scale the manner in which the shape of important replacement 
deposits may be influenced by preexisting fissures. 

As is explained on page 164, the pyrite in the district was not all formed at the same time, 
but some ores show at least two generations. 

OXIDES. 

QTIAATZ. 

The very common mineral quartz is not an important gangue material in the Breckenridge 
ore deposits, and the nonsiliceous character of most of the veins is one cause of their failure to 
outcrop. In the veins carrying much galena and sphalerite even occasional bunches of ordinary 
vein quartz are rare. None was noted in the Country Boy vein and only a very little, crystalUzed 
with sulpliides and siderite {or a related carbonate), was found in the Wellington vein. In the 
I. X. L. mine the ore consists of shattered quartz monzonite porphyry and quartzite with the 
interstices filled, or partly filled, with pyrite, sphalerite, galena, chalcopyrite, siderite, and 
quartz- The quartz b in part irregularly intercrystalhzed with the sulphides and in part incrusts 
the walls of the vugs in transparent projecting crystals of the usual form. The presence of the 
quartzite liere had some influence in inducing the crystallization of the quartz and may have 
furnished some of the sihca. In the Jessie mine there is a httle milky quartz deposited on the 




86 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

walls of the countless small fissures in quartz monzonite porphyry that carry the sulphides. 
The quartz is not in well-formed crystals and does not project far from the walls. Many of these 
shapeless blebs of quartz appear to have grown upon parts of quartz phenocrysts cut by the 
fissure, the spots of quartz in the walls inducing the deposition of silica from therein solutions. 

In the ore bodies of the Hamilton mine, which are in quartz monzonite porphyry similar 
to that at the Jessie, the sulphides, especially the pyrite, are associated with some quartz gangue 
which in part represents the silification of crushed porphyry. 

In the Puzzle and Gold Dust veins, which traverse porphjrry, quartzite, and shale, the ore 
contains some quartz in the form of druses oi> cementing and partly replacing crushed rock that 
apparently was in the fissures when ore deposition began. 

Ores more siliceous than those in the vicinity of Breckenridge occur in the pre-Cambrian 
rocks near the sources of the Blue, and the sulphides of the Senator and Arctic mines are in a 
matrix of massive vein quartz, associated in some places with a little carbonate similar to that 
described in the veins near Breckenridge. 

Quartz is a conspicuous mineral in the quartz monzonite porphyries, occurring as bipyram- 
idal phenocrysts, some of which are sharply bounded by crystal planes, but most of which 
have undergone more or less magmatic corrosion, as shown in figure 2 (p. 44). 

Quartz in detrital grains is the chief material of the sandstones of the region, especially of 
the '\Sawatch*' and Dakota quartzites, in which the cement also is quartz. 

HEMATITE. 

The specular variety of the anhydrous ferric oxide, or specularite, occurs in small quantity 
with garnet, calcite, epidote, amphibole, and pyrite in altered sediments on the south slope of 
Gibson Hill. It is a microscopic constituent of the ore of the Sultana mine at the west base of 
the same hill, as shown in Plate XXII, J5. Its associates here are magnetite, pyrite, chlorite, 
quartz, and some minute needles that are probably amphibole. Specularite occurs also with 
pyrite, sphalerite, magnetite, chalcop^rite, and small quantities of galena in the Witch Hazel 
vein of the Senator mine at the head of the Blue. (See PL XXXI, J5, p. 160.) Massive hematite 
with magnetite is abundant in the dumps of some old abandoned workings in the ''Sawatch" 
quartzite west of Hoosier Pass. The coloring matter of the bright-red '* Wyoming'' formation 
is probably hematite in greater part. 

MAGNETITE. 

Magnetite, or one of its alteration products, pyrite and limonite, is everywhere present in 
the igneous rocks as a microscopic constituent and in many places persists in the soils and sedi- 
mentary rocks derived from igneous rocks, as shown by the black sands obtained in panning. 
In some of the pre-Cambrian pegmatites it is a fairly abundant megascopic mineral, as may be 
seen on the headwaters of the Blue or at Swandyke, east of the Breckeiwidge district. It occurs 
in some of the metamorphic rocks described on pages 93-94 and in Prospect Gulch has been found 
in fine granular form in a matrix of earthy calcite. At the Senator mine it occurs intimately 
associated with specularite in an ore containing quartz, chlorite, pyrite, specularite, and 
chalcopyrite. (See PI. XXXI, B, p. 160.) In massive form, mixed with hematite, it occurs 
in the upper part of veins worked many years ago in the Cambrian quartzite on North Star 
Mountain, west of Hoosier Pass. The dump of the Old Ironsides mine on Nigger Hill shows 
some magnetite in oxidized ore, associated with limonite and malachite. The n[iineral here 
appears to have been formed during the oxidation of the ore. 

LIMONITE. 

The hydrous ferric oxide is the usual product of the oxidation of pyrite and is found in 
the upper parts of all ore deposits in the district. It is the principal matrix of much of the 
crystalline gold of Famcomb Hill. 



MINEBALOGY. 87 

CARBONATES. 
CAIiCITE. 

Calcite, so far as known, does not occur in large crystals in the vicinity of Breckenridge, 
but in massive form it is the chief constituent of most of the limestones and as a microscopic 
constituent is present in nearly allof the monzonite porphyry as exposed at the surface, accom- 
panied by chlorite, quartz, and epidote. It occurs nearly pure with garnet, epidote, and sulphides 
in metamorphosed calcareous sediments, as described on pages 93-94. It forms unimportant 
stalactitic crusts in the ore of the Wellington mine and in the Little Sallie Barber mine, where it 
is younger than siderite. In various states of impurity through admixture of other isomorphous 
carbonates it is present in small quantities in fissures and vugs in most of the lead-zinc ores. 
It constitutes the principal gangue mineral in the deeper parts of some of the narrow gold veins 
of Famcomb Hill and in ummportant sulphide-bearing stringers elsewhere in the district. In 
the Bowery tuxmel, near Lincoln, calcite fills vugs in iron-magnesium-manganese carbonate 
carrying pyrite, galena, and sphalerite. In a prospect in Prospect Gulch it forms a compact, 
rather earthy mixture with magnetite. Finally, it is a conmion cementing material in the 
various sandstones and in microscopic or inconspicuous form is widely distributed through 
nearly all the rocks and ores in the district. 

SIDEBITE. 

Pure siderite, so far as known, does not occur in the Breckenridge district. Most of the lead 
and zinc, however, contain subordinate quantities of white to pale buff, brown, or pink carbonate 
which chemical tests show to consist of the carbonates of iron, manganese, magnesium, and 
calcium in various proportions. Much of this material closely resembles dolomite in color, 
hardness, crystallization, and behavior with dilute acid, but whether any of it really belongs to 
this species is doubtful. The rhombohedral carbonates of the bases above mentioned constitute 
an isomorphous series, and apparently any mixture of them is capable of crystallizing as a single 
mineral not necessarily belonging to a recognized species. The essential fact to be recorded is 
that most of them are carbonates of iron, magnesium, calcium, and manganese, the quantities 
of these constituents generally being in decreasing order as named. Most are probably nearer 
to siderite or to the subspecies ankerite than to any other. 

The siderite occurs as the filling of vugs or as small irregular veinlets in the sulphides of the 
lead-zinc ores, such as those of the Wellington and Country Boy mines. It is not altogether 
absent from deposits, like that of the Jessie mine, in the siliceous quartz monzonite porphyry, 
but it is not nearly so abundant in those as in the veins traversing the more femic monzonite 
and diorite porphyry. In some places it forms botryoidal incrustations on the sphalerite or 
other sulphides, the free surface of the crust showing the closely crowded points of small, im- 
perfect rhombohedral crystals. (See PI. XXV, p. 124.) In most of the ore bodies the carbon- 
ate is wholly subordinate to the sulphides, but in parts of some veins — the Sallie Barber, for 
example — similar material constitutes an abundant gangue tliat incloses a smaller quantity 
of sulphides. Much of the carbonate has a pinkish tint, due to the presence of the rhodochrosite 
or manganese carbonate molecule, and in some of the ore of the Wire Patch mine this color is 
so pronounced as to indicate a considerable proportion, perhaps over 25 per cent, of manganese 
carbonate. A qualitative chemical examination of this pink carbonate by W. T. Schaller 
showed much iron and manganese, with only a little calcium and magnesium. It possibly might 
be classed as a ferruginous rhodochrosite. 

Some of the carbonate mixtures, which for lack of a more appropriate name are here 
grouped under siderite, are comparatively hard, coarsely crystalline aggregates that on frac- 
ture show broad curved cleavage faces; others are pulverulent or earthy. As described in 
detail in Chapter VII, the characteristic wall-rock alteration alongside of the sphaleritic veins 
of the district consists in great part in the introduction into the rock of a mixed carbonate, 
predominantly siderite. Some of the ferruginous carbonate, such as that present in vugs in 
the I. X. L. ore body, dissolves freely in lumps in cold dilute acid, and might therefore be mis- 
taken for an impure calcite; but chemical tests show it to contain much iron and magnesiimi 
as well as calcite. It is probably near ankerite in composition. 



88 GEOLOGY AND OEE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

SMTTHSONITE. 

An impure zinc carbonate containing ferrous carbonate and a trace of manganese, known 
to the miners as '^dry bone/^ occurs as minute veinlets and as rough spongy bunches and 
incrustations of yellowish-white color in the sphalerite of the Sallie Barber mine, as illustrated 
in Plate XXVI, A (p. 126). The mineral was noted also in less abimdance in the Wellington 
mine and is probably present in the upper parts of most of the sphaleritic ore bodies. The smith- 
sonite forms below the water level under conditions that permit also the contemporaneous 
deposition of minute crystals of pyrite. It appears that acid sulphate solutions from the 
overlying zone of general oxidation attacked particularly the siderite associated with the 
sphalerite, setting free carbon dioxide, which, in turn, reacted with zinc sulphate to form 
smithsonite. At the same time the sulphate radicle was reduced, at least in part, and the 
resulting sulphur combined with iron set free from the siderite to form pyrite. The highly 
cavernous texture of the resulting smithsonite shows also that in this process, the forerunner 
of general oxidation, some material, probably both sphalerite and siderite, was removed from 
the ore by solution. 

CEBTJSITE. 

The carbonate of lead formed an important part of many of the lead-silver ores formerly 
shipped, and a little may stiU be seen in the upper workings of the Wellington, Minnie, Dunkin, 
and other mines in the southern half of the district. Good crystals, however, are rare and 
the carbonate ore is generally soft and earthy. Masses of crystalline material at all com- 
parable to those found in the upper parts of the Hercules and other lead-silver mines of the 
Coeur d'Alene district, in Idaho, have never been foimd at Breckenridge. 

MAULCHTTE. 

There are no important occurrences of the green carbonate of copper in the district. It 
is formed in very small quantities in the oxidation of ores containing chalcopyrite on Fam- 
comb and Nigger hills, in Muggins Gulch, and perhaps elsewhere. 

AZT7BITE. 

The blue carbonate of copper occurs sparingly in some of the oxidized ores derived from 
sulphide mixtures containing chalcopyrite. It was noted in the D\mkin workings, on the 
south slope of Nigger Hill, associated with cerusite. 

SILICATES. 
OBTHOCLASE. 

The most conspicuous mode of occurrence of orthoclase in the Breckenridge district is 
as the large phenocrysts of the quartz monzonite porphyry, described on pages 44-50 and 
illustrated in Plate VI, A (p. 44). Some of those are as much as 3 inches in length, and they 
are generally bounded by regular crystal planes. In some places where the porphyry is 
weathered or softened by vein solutions the crystals resist alteration better than the rest 
of the rock and may readily be picked from their soft matrix or from the residual soil into 
wliich the rock has crumbled. Some of these are short, stout, simple crystals, elongated 
parallel with the clinoaxis and boimded dominantly by the basal pinacoid (001), the clino- 
pinacoid * (010), the orthodome (201), and the prism (110), generally with the modifying faces 
of the unit hemipyramid (IlO), the clinodome (021), and the prism (130). Others, elon- 
gated, as a rule, parallel with the prism axis, are Carlsbad twins, and mostly show the same 
faces listed for the stouter untwinned crystals. These common habits of the orthoclase pheno- 
crysts are nearly the same as those illustrated in figures 9 and 11 on page 317 of Dana^s 
'* System of mineralogy." 

> For convenience of reference to Dana, the monoclinic terminology is retained. Some later works, however, treat orthoclase as triclinia with 
paeadomonoclinic symmetry. 



GEOIiOGY AND ORE DEPOSITS OF BBECKENBIDGE DISTRICT, COLORADO. 89 

• 

The orthoclase of the porphyries is not, as a rule, perthitic, although most of the larger 
crystals mclude a few smaller ones of plagioclase. In anhedral form it is an abundant con- 
stituent of the monzonitic porphyries, especially as microscopic grains in the groundmass. 

Neither ordinary orthoclase nor the variety adularia has been detected as a product of 
ore deposition in this district. 

MICBOCLINE. 

Microcline is common in the pre-Cambrian crystalline rocks and in the sediments derived 
from them. No special study was made of it. 

ANDESINE AND LABBADOKITE. 

The two plagioclases, andesine and labradorite, as described in Chapter III (pp. 43-62), 
are essential constituents of the porphyries. Most of the plagioclases of these rocks are near 
the dividing line between andesine and labradorite. 



Orthorhombic pyroxene, probably hypersthene, is a minor original constituent of some of 
the dioritic porphyries and was observed microscopically in some of the fresher specimens from 
the Wellington mine. It readily undergoes decomposition and is not likely to be found in 
any but the freshest of the more femic porphyries as exposed in deep tunnels or crosscuts. It 
presents no unusual features. 

ATTGITE AND DIOPSIDE. 

- A nearly colorless monoclinic pjrroxene is a constituent of some of the fresher and more 
femic monzonite porphyries. It is rarely present in specimens collected from natural out- 
crops, owing to its ready decomposition to calcite, chlorite, and epidote. 



Common hornblende or one of its alteration products is a constituent of the monzonitic 
porphyries, as described in Chapter III (pp. 51-53), where some of its peculiarities are discussed. 
A dark-green amphibole occurs also in gametized sedimentary rocks on the south slope of Gibson 
Hill, as noted on page 93, and a similar mineral in very small acicular crystals is a constituent 
of the ore of the Sultana and neighboring tunnels. 

OABNET. 

A brown garnet is fairly abundant at many locaUties in the district where metamorphism 
of the kind described on pages 93-94 has taken place, especially on the south slope of Gibson 
Hill, between Summit and Browns gulches, and in the vicinity of the Nebraska prospect, on the 
south side of French Gulch. No large, well-formed cry tals have been seen. The material is granu- 
lar, some being rather friable, but most of it being a hard, compact garnet rock. No chemical 
analysis of this material has been made, and it is not known whether it is essentially a calcium- 
aluminum garnet (grossularite) or a calcium-iron garnet (andradite). The garnet is generally 
associated with calcite, epidote, quartz, and sulpliides, especially with pyrite, chalcopyrite, and 
sphalerite. Less common or abundant associated minerals are specularite and amphibole. 
Garnet is often puzzling to prospectors, who sometimes mistake it for cassiterite or "tin," 
sometimes for a mineral supppsed to contain rare earths. 

Under the microscope most of the Breckenridge garnet shows anomalous birefringence. 

znicoN. 

Zircon occurs in the Breckenridge district only as an unimportant microscopic constituent 
of the porphyries. 



90 GEOLOGY AND OBE DEPOSITS OF BBECKENBIDGE DISTRICT, COLORADO. 

EPIDOTE. 

Epidote, associated with chlorite and calcite, is of common occurrence, chiefly as a micro- 
scopic constituent, in the porphyries^ where it has resulted from the alteration of the femic 
minerals. It is present also in sediments modified by contact metamorpliism, as described on 
pages 93-94. Here its association is commonly with garnet, calcite, hematite, and sulpliides. 

ALLANITE. 

The occurrence of allanite as an accessory constituent of the porphyries is described in 
Chapter III (pp. 44-50). It is nowhere abundant and has been seen only in small crystals, 
generally anhedral. 

MUSCOVITE (SEBICITE). 

The minutely foliated variety of muscovite known as sericite is a common product of the 
alteration of rocks by vein-forming solutions as described on pages 94-99. It is especially abun- 
dant in the altered quartz monzonite porphyry, as, for example, near the Jessie mine. The fine 
scales of this mineral are microscopic in size. and present no exceptional characteristics. • 

Ordinary muscovite in the form of detrital flakes is very abundant in the pre-Dakota 
sediments. It is an important constituent also of the pre-Cambrian schists, gneisses, and 
granites whence the micaceous sediments were derived. 

BIOTTTE. 

Common black mica is an important mineral in the monzonitic porphyries. In many of 
these it is altered to chlorite and epidote. Further notes on its characteristics will be found in 
Chapter III (p. 51). 

CHLOKITE. • 

Chlorite is a common alteration product of the femic minerals in the porphyries of the 
district and gives to these rocks, especially to the calcic varieties, the greenish color character- 
istic of their surface exposures. It is not as a rule closely associated with the ores except those 
intimately connected with the type of metamorphism described on pages 93-94, although it is 
present in the veins of the Senator mine associated with quartz, pyrite, specularite, and carbon- 
ate, as noted on page 159 and shown in Plate XXXI, B (p. 160). In the ore of the Sultana 
and Fox Lake mines chlorite is an abundant microscopical constituent associated with quartz, 
calcite, pyrite, hematite, and magnetite. (See PI. XXII, 5, p. 92.) 

EAOIilNITE. 

Kaolinite is nowhere abundant in association with the ores. It occurs, however, as a 
product of weathering in some of the poq)hyries, particularly those in which sericite has been 
formed. 

CHRYSOCOLLA. 

The hydrous silicate of copper does not occur in any important quantity in the Brecken- 
ridge district. A very little of this mineral was noted in some oxidized lead-silver ore from 
lessee workings on the ground of the Dunkin Alining Co., on Nigger Hill. 

TITANOSILICATES. 

TITANITE. 

Titanite is a fairly abundant microscopical constituent of the porphyries, particularly of 
those in which the crystallization approaches the granitic texture. 

PHOSPHATES. 

APATITE. 

Apatite is known in the Breckenridge district only as a minor microscopic constituent of 
the igneous rocks. 



MINERALOGY. 91 

SULPHATES. 

BABITE. 

Barite is not a eommon mineral in the Breckenridge ores, but occurs sparingly in aggre- 
gates of imperfect platy crystals in small veinlets in the sulphides of the Wellington mine and 
in similar crystals associated with siderite and implanted on pyrite and sphalerite in the Rose 
of Breckenridge mine. Its formation was later than that of the mass of the ore in which it 
occurs. 

OYPSTJM. 

In pulverulent form, the hydrous sulphate of calcium was said hy a prospector, who brought 
some in for determination, to be found in small quantities in a prospect tunnel near the head of 
French Gulch, as a sediment left by the melting of ice during the summer. This mineral occurs 
in small quantities also in the black shale of the Gold Dust claim, in Dry Gulch, and as films or 
flat-stellate clusters of minute crystals in the cleavage planes of the shales of Famcomb Hill, 
In all three locaUties it is probably formed by the action of the sulphuric acid from weathering 
pyrite upon calcium carbonate in the adjacent rocks. 



PLATE XXII. 
Photomicrographs of Metamorphosed Sedimentary Rooks. 

A. Gramet-epidote rock with sulphides. South slope of Gibson Hill. X 40. 

Described on page 93. py, Pyrite; g, garnet. Most of the rock is a fine mixture of garnet, epidote, calcite, 
and quartz. The pyrite is in part at least younger than the garnet and is contemporaneous with the calcite. ^ 

B. Ore. Fox Lake tunnel, west base of Gibson Hill. X 40. Nicols crossed. 

Described on page 94. Section shows chiefly specularite (s), quartz (q), calcite (ca), and chlorite (chl). With 
these is commonly much pyrite. The rock is decidedly magnetic and some of the opaque grains are presumably 
magnetite. 

92 



U. S. OEOLOOICAL aURVEV 




PHOTOMICROGRAPHS OF METAMORPHOSED SEDIMENTARY ROCKS. 



CHAPTER VII. 
METAMORPHISM. 

SCOPE OF CHAPTER. 

Under the heading '^Metamorphism" will be described alterations in the rocks due to 
igneous intrusion, ore deposition, and deep weathering, especially those changes effected in the 
wall rock by ore-bearing solutions. Most of the pre-Cambrian rocks also are metamorphic, 
but their alteration dates from a period long anterior to the formation of the ores, and was 
effected by forces active over regions so wide that discussion of these ancient changes would 
be inappropriate in a study concerned, like the present one, primarily with the later chapters 
in the geologic history of a small area. 

METAMORPHISM CONNECTED WITH THE PORPHYRY INTRUSIONS. 

The larger masses of intrusive porphyry, as shown in Chapter IV (pp. 63-71), in spite of 
their great irregularity, have generally the form of sheets or sills. It is well known that intru- 
sive bodies of this kind are not ordinarily associated with pronounced contact metamorphism, 
and the porphyries of Breckenridge are no exception to this general rule. The rock alongside 
the eruptive may be shattered and, where contacts are exposed, may show some variation 
from its normal color or hardness, but there is rarely any development of new minerals or 
textures. Nevertheless, although the porphyry masses are not uniformly bordered by meta- 
morphosed rock, there are places in the district where the sedimentary rocks have undergone 
modification of a kind usually regarded as due to igneous intrusion. These local areas of meta- 
morphism, however, are not necessarily in contact with porphyry, so far as surface exposures 
show, and are not all demonstrably contact effects. They are thought to mark places where, 
during the epoch of intrusion, hot vapors or solutions, escaping perhaps not from the now visi- 
ble sheets but from deeper magmatic sources, found their way through the sedimentary beds 
and reacted upon certain strata that were particularly susceptible to metamorphism. Between 
these solutions and their effects on the one hand and ore deposition on the other, there is 
probably no definite line of demarcation. 

One place illustrating this local metamorphism is the south slope of Gibson Hill, where, 
in beds belonging to the Dakota, some blanket-like ore deposits, formerly worked on a small 
scale, are associated with bunches and lenses of gametized rock. One mass of gamet-epidote 
rock containing a little specularite outcrops on the road a few hundred yards west of Gibson 
Gulch. Its stratigraphic relations are not clear, but it probably was at one time a lenticular 
bed of impure limestone in the quartzite. Lower down the slope dumps of the abandoned 
BulUon King and adjacent tunnels, which cut beds only a short distance above the pre- 
Cambrian and possibly ''Wyoming" rather than Dakota, contain much gametized material. 
For the most part this is altered calcareous shale and all gradations may be observed between 
hard cherty gray-green shale, spotted here and there by small bimches of garnet, to soUd masses 
of brown gamet-epidote rock which, together with these two predominant minerals, contains 
some calcite and nests of dark-green amphibole in slender prisms, with sulphides, mainly 
pyrite and chalcopyrite. The microscope shows that the sulphides probably crystallize a little 
later than most of the silicates. (See PI. XXII, A, p. 92.) Associated with the gametized 
shale on the dumps is some dark micaceous shale, which is not noticeably metamorphosed, 

93 



94 GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

and some pebbly quartzite, which overlies the shale and contains some garnet. On the whole, the 
material on these dumps indicates metamorphism of considerable local intensity, but there is 
no contiguous body of porphyry visible to whose action it can be directly assigned. 

A similar gametized calcareous shale occurs between Summit and Brown gulches in a 
mass of Upper Cretaceous shale that is surrounded by quartz monzonite porphyry. Here there 
is no difficulty in accounting for the metamorphism as in general an effect of the intrusion of 
the porphyry, although it is not clear why this particular locality should exhibit more change 
than similar shale elsewhere shows close to the same porphyry. The explanation probably is 
that the metamorphism described is not so much a function of mere proximity to the porphyry 
as it is of channels or courses along which emanations from the solidifying igneous masses 
escaped, and of the chemical character of the rock affected. 

The nearly horizontal blanket deposits exploited in past years in the Sultana, Fox Lake, 
and other tunnels along the west base of Gibson Hill are associated with decided metamor- 
phism of the ^^ Wyoming'' beds in which they occur, just above the pre-Cambrian. The obvious 
results of the metamorphism are a dark-green color and an abundance of disseminated pyrite. 
The microscope shows that the arkosic grits or sandstones have been changed to almost wholly 
recrystallized aggregates of quartz, calcite (probably wiih admixture of other isomorphous 
carbonates), chlorite, pyrite, and, in some places, magnetite. The magnetite, while partly in 
the usual granular form, occurs also in clustered lamellar aggregates intergrown with the calcite 
and quartz, as shown in Plate XXII, B (p. 92). Where the magnetite is abundant the altered 
rock is nearly black. 

Another locality of decided local metamorphism is on the north side of Black Gulch, a 
ravine opening into French Gulch above Lincoln. Here, as shown in the workings of the 
Nebraska mine, some calcareous beds in the Upper Cretaceous shale are altered in part to 
garnet, calcite, and pyrite; in part to dark, compact, heavy material containing much mag- 
netite and pyrite; and in part to pale green-gray clierty rocks containing garnet, epidote, 
chlorite, calcite, quartz, specularite, and pyrite in various proportions. The pyrite is partly 
disseminated and partly crystallized with chalcopyrite in small irregular veinlets. Some of 
the dark material rich in magnetite has been shipped as a gold ore. 

Other examples of metamorphism similar to those described might be cited from different 
parts of the district, but the occurrences mentioned are thought to be sufficiently illustrative 
of this sporadic kind of alteration. 

METASOMATIC CHANGES IN WAIiIj ROCK OF ORE DEPOSITS. 

GENERAL CONSIDEBATIONS. 

That the occurrence of ore is generally associated with changes in the character and appear- 
ance of the adjacent rock is a widely recognized fact and one often turned to use by the pros- 
pector, who learns to avail himself of the visible signs of alteration as an aid in his search. 
Ordinarily the change is most noticeable in igneous rocks, particularly in that very large class 
to which the presence of femic (ferromagnesian) siUcates imparts originally some shade of 
gray. Under the influence of solutions active in ore deposition these dark silicates are as a 
rule decomposed, their iron in part recombines as pyrite, light-colored minerals such as seri- 
cite, quartz, carbonates of the alkaline earths, and alunite are formed, and the rock so changed 
is generally softer and of lighter color than before. Even where the alteration results in no 
conspicuous modification of color and texture, it may nevertheless often be detected by chem- 
ical and microscopical means, for there are few wall rocks chemically so inert as not to be 
influenced in some degree by the reactions involved in the transportation and precipitation 
of ore constituents. The nature of this effect affords one way of learning something of the 
character of these solutions, for it is as surely a record of their work as is the ore itself. In 
this branch of investigation, however, it is necessarj^, just as in the study of the ores, to guard 
against crediting a certain result to a single process when in reality it may record the super- 
position of a later process, such as downward enrichment, upon another and earlier deposition. 



METAMORPHISM. 96 

ALTEBATION OF MONZONTTE POBPHYBY OF THE WELUNOTON MINE. 

SELECTION OF MATERIAL. 

In the Wellington mine, near the east end of the Oro level, there was in 1909 a good oppor- 
tunity of studying the alteration of the country rock near the east vein, which is reached 
from the hanging-wall side by a crosscut from the fresh diorite porphyry (Br. 217) described 
with a chemical analysis in Chapter III (p. 55). The part of the level whence the specimen 
came is about 350 feet below the surface. Although this is beyond the reach of oxidizing 
water, specimens from greater depth would have been preferable for this purpose could they 
have been obtained in the district. On the other hand, decidedly favorable features of this 
locaUty for comparisons of the kind to be undertaken are the very uniform character of the 
fine-grained porphyry and the opportunity of studying changes and collecting specimens along 
a single crosscut which passes directly from fresh rock into the vein, here about 5 feet wide, 
with fairly definite walls along which there is not much gouge. The vein of this place, more- 
over, contains only a small quantity of galena and does not appear to have been notably 
enriched by downward-moving solutions. The specimen (Br. 217) of which an analysis and 
description have been given in Chapter III (p. 55) was taken 25 feet from the vein. The 
constituent minerals are almost perfectly fresh, although there is a Uttle carbonate along micro- 
scopic fractures. 

At a distance of 10 feet from the vein another specimen (Br. 215) was collected. At 20 
feet from the vein the rock is still dark gray and essentially unaltered; but from there on it 
becomes lighter in shade, and at 10 feet is pale gray to buff in color, shows no dark siUcates, 
t^nd contains much very finely disseminated pyrite in crystals barely visible with the naked 
eye. Minute fissures (avoided in preparing the sample for analysis) contain pyrite, sphalerite, 
and galena, and here and there a speck of sphalerite can be detected with a lens in the body 
of the rock. Under the microscope the rock, while retaining faint traces of its original texture, 
is seen to have been changed into a fine aggregate of carbonate, sericite, and quartz, with 
a little pyrite and sphalerite. The only original constituent remaining is apatite. The car- 
bonate is not calcite, but has the appearance of siderite or dolomite. There are also some 
specks of cloudy yellowish material resembling leucoxene. 

Another specimen (Br. 214), collected a few inches only from the vein, is light gray and 
shows abundant tiny specks of pyrite, sphalerite, and galena disseminated throughout its mass 
and disposed along microscopic fractures. Under the microscope this rock diflFers from the 
one just described only in the greater abundance of the sulphides. (See PL IX, p. 52.) 



96 



GEOLOGY AND ORE DEPOSITS OF BB^CKENRIDGE DISTRICT, COLORADO. 



COMPABISON OF CHEMICAL ANALYSES. 



Chemical analysis shows the three varieties of rock to have the compositions given in 
columns 1, 2, and 3 of the following table: 

Table showing chemical alteration ofdiorite porphyry. 





l'- 


2 " 


1 
3 


•«( 


2a. 

1 « 


Ba 

* 


4a 


4b 


5a 


5b 


6a 


6b 


sio,..".; 


57.35 

16.29 

3.15 

4.36 

2.41 

5.66 

4.50 

3.39 

.15 

.70 

1.07 

Trace. 

.46 

.70 

.09 

None. 


49.59 

14.91 

.52 

10.46 

2.02 

1.96 

1.33 

3.51 

.16 

3.17 

1.03 

None. 

9.40 

.47 

.36 

Trace. 

Trace. 


46.62 

12.66 

Trace. 

o 11. 15 

4.02 

1.55 

1.35 

1.68 

.31 

3.41 

1.01 

None. 

11.48 

.50 

1.99 

.97 

.52 

None. 


158.46 
45.01 

8.70 
12.05 

6.66 
15.64 
12.43 

9.37 
.41 

1.93 

2.96 


141.68 

42.60 

1.48 

29.88 

5.77 

5.60 

3.80 

10.03 

.46 

9.06 

2.94 


136.59 
37.09 


-16.78 

- 2.41 

- 7.22 
+17.83 

- .89 
-10.04 

- 8.63 
+ .66 
+ .05 
+ 7.13 

- .02 


-21.87 

- 7.92 

- 8.70 
+20.61 
+ 5.12 
-11.10 

- 8.48 

- 4.45 
+ .50 
+ 8.06 


+ 

+ 
+ 
+ 


10.59 

5.35 

91.90 

147,97 

13.34 

64.19 

69.43 

7.04 

12.19 

369.43 

.67 


+ 
+ 

+ 
+ 


13.80 

17.60 

100.00 

171.04 

76.88 

70.97 

68.22 

47.49 

121.95 

417.61 


-6.04 

- .87 
-2.60 
+6.42 

- .32 
-3.61 
-3.11 
+ .24 
+ .02 
+2.54 


- 7.87 


AliOi...- 


- 2.85 


FOiOt 


- 3.13 


FeO . . .i 


32.66 
11.78 
4.54 
3.95 
4.92 
.91 
9.99 
2.96 


+ 7.42 


MeCx 


+ 1.84 


CaO 


- 4.00 


NaiO 


- 3.05 


KiO 


- 1.60 


HtO-.i 


+ .18 


H»0+ . .4 


+ 2.90 


TiOi 




ZrOj 










COj....:t 


1.27 

1.93 

.25 


26.86 
1.34 
1.03 


33.63 
1.46 
5.83 
2.84 
1.52 


+25.59 
- .69 
+ .78 


+32.36 
- .47 
+ 5.58 
+ 2.84 
+ 1.52 


+2,014.96 
- 30.57 
+ 312.00 


+2,548.03 
- 24.35 
+2,232.00 


+9.21 
- .21 
+ .28 


+11.65 


PjOs....; 


— .17 


FeSj- . . .i 


+ 2.01 


ZnS J. 


+ 1.02 


PbS : 














+ .55 


CrtOi 


None. 

Trace. 

None. 
.12 
.10 
.05 

None. 

None. 
















ViOs 






















NIO 


None. 

1.10 

.07 

Trace. 

None. 


None. 
.92 
None. 
None. 
None. 




















MnO...'::. 


.33 
.27 
.14 


3.14 
.20 


2.69 


+ 2.81 

- .07 

- .14 


+ 2.36 

- .27 

- .14 


+ 


851.52 

25.92 

100.00 


+ 


715.15 
100.00 
100.00 


+1.01 

- .02 

- .04 


+ .86 


BaO 


- .10 


BrO 




— .06 


LltO - 








Rfu*e e*i^rth8. 














































100.55 


100.06 


100.12 


277.81 


285.87 


293.36 


+ 8.06 


+15.55 






+2.90 


+ 5.60 










Specific gravity: 

Mfiffi. 


2.763 
2.799 


2.857 
2.874 


2.930 
2.940 




















Powder^ 









a Uncertain owing to presence of ZnS. 



& At 25^^ C. crushed to go through 100-mesh. 



1. Chemical analysis of fresh diorite porphvry, 25 feet from vein, Wellington mine. W. T. Schaller, analyst. 

2. Chemical analysis of altered diorite porphyry, 10 feet from vein, Wellington mine. W. T. Schaller, analyst. 

3. Chemical analysis of altered diorite porphyry, less than 6 inches from vein, Wellington mine. W. T. Schaller, analyst, 
la. Constituents in grams in 100 cubic centimeters of fresh rock. 

2a. Constituents in grams in 100 cubic centimeters of partly altered rock. 

3a. Constituents in grams in 100 cubic centimeters of most altered rock. 

4a. Gain or loss in grams of each constituent in the first stage of alteration of 100 cubic centimeters of fresh rock. 

4b. Total gain or loss in grams of each constituent at the complete alteration of 100 cubic centimeters of fresh rock. 

5. Gain or loss in percentage of original mass of each constituent— a, in partly altered rock; b, in fully altered rock. 

6. Gain or loss in percentage of total original rock mass— a, in partly altered rock; b, in fully altered rock. 

Direct comparison of analyses of fresh and altered rock does not as a rule show accurately 
what has taken place during alteration; it is necessary to take into account the density or spe- 
cific gravity and the porosity of the rocks compared. An analysis, for example, may be regarded 
as representing in grams the weight of each constituent in a mass of rock weighing 100 grams. 
Obviously if the altered rock, to take an extreme case, is only half as dense as the fresh rock, 
the analysis of the altered rock will give the constituents contained in a mass of twice the volume 
of the initial rock or will represent the alteration of a mass of fresh rock weighing 200 instead 
of 100 grams. Consequently to show the actual gains and losses of the several constituents the 
percentage figures in the analysis of the altered rock would have to be halved in order to com- 
pare them with the corresponding figures in the original analysis of fresh rock. So great a 
difference in density as that postulated for the sake of illustration scarcely ever exists; neverthe- 
less the difference is rarely negligible.* 

There is no evidence that the present alteration has caused any general swelling or contrac- 
tion of the rock mass as a whole. Comparison of the determinations of specific gravity made 
with hand specimens and with rock powder shows that the rock becomes slightly less porous 
and decidedly heavier with the progress of alteration. Of the fresh rock a fragment weighing 

1 For a full and excellent discussion of this subject see Lindgren, Waldemar, Metasomatic processes in fissure veins: Trans. Am. Inst. Min. 
Eng., vol. 30, 1901, pp. 591-601. Throughout this paper ''cubic meter" should read ''cubic decimeter." 



METAMORPHISM. 



97 



219.69 grams in air has a specific gravity of 2763 as a whole and of 2.799 in powder. A simple 
calculation from these data gives a pore space of 1.4 per cent. Although the rock is not notice- 
ably porous as seen under the microscope, the actual percentage of submicroscopic pore space is 
probably a little greater than that found, as not all the pores are opened by crushing the rock 




26 25 24. 23 22 21 



20 19 16 17 16 15 14 13 12 n 10 9 8 7 6 5 

ABSCISSAS REPRESENT DISTANCE FROM VEIN IN FEET 



Figure 10.— Diagram showing alteration of diorite porphyry by vein-forming solutions. 

to powder. Of the altered rock 10 feet away from the vein a fragment weighing 402.37 grams 
in air has a specific gravity of 2.857 as a whole and of 2.874 in powder. These figures give a 
porosity of 0.5 per cent. Of the altered rock close to the vein a fragment weighing 169.11 grams 
in air has a specific gravity of 2.930 as a whole and of 2.940 in powder. From this is deducible 
a porosity of a little over 0.3 per cent. 

90047°--No. 75-11 7 



98 



GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 



The most instructive kind of comparison between fresh rock and altered rock is one that will 
show quantitatively what constituents have been added to or subtracted from a known volume 
of the fresh material in the course of the change. Chemical analyses as ordinarily expressed 
represent the composition of equal weights of material. Inasmuch as there has in this case 
been no perceptible change in total volume and as the summation of each analysis is very close 
to 100, the multipUcation of the figures of each analysis by the figures representing the specific 



MnO 



PbS 



ZnS 



SiOa 


— + 


\% 


-^ + 


11 


" — + 


10 ^^ 


^^ + 


9 ^ 


— + 


8 


 — + 


7 ^ 


 — + 


6  — 


 + 


5  


" — + 


4 " 


+ 


3  


- — + 


2  


4- 


I . 


■-frro 


"rT' 


iiiiili 


2 

3 -- 

6 


xV""^^^*" 


6?S< 


v.-— \ _5 


*x^r 


^y^^^"^ 





AlsOa 



FeaOa 



PeO 



FeS2 



00 



MS 



CO 



|e>|eo|'^|io(gott>»| 



< I > I I I > > 



|ao t^ « ud ^ 00 


w 




\ 


CO 


\ 


00 


1 


2 






rr 1 1 1 1 1 

'4J U 




1 


+ 


1 


+ 

1 


+ 


+ 
i 


+ 


■-F 


^ 



555 MgO 



02^ 



Ti02 



Ha0+ 



Qii 

' • 

- — a.. 

:3i 



CaO 



Na2 



K2O 



H2O- 

FiauRE 11.— Diagram illustrating gains and losses of each chemical constituent In terms of percentage of mass of original fresh diorite porphyry. 
Radial distances between successive circles correspond to 1 per cent. Gains are outside and losses inside of the heavy circle representing fresh 
diorite porphyry. Dash-lined angular outline represents partly altered porphyr>'. Full-lined outline represents fully altered porphyry. Areas 
have no significance. 

gravities of the respective rock masses will give approximately the weight in grams of each 
constituent contained in 100 cubic centimeters of each of the three varieties of rock. The results 
are shown in columns la, 2a, and 3a of the table on page 96. 

The gains and losses arrived at in columns 6a and 6b may be applied directly by addition 
or subtraction to the figures of the chemical analysis of the fresh rock, and this, as shown by 
the following table, perhaps will express the nature of the change more clearly than is done by 
the larger table. 



METAMOBPHISM. 



99 



Table showing 


the alteration ofdiorite porphyry in percentage of original rock 


mas9. 








1 2 3 

1 




1 


2 


3 


SiOi 


57.35 

16.29 

3.15 

4.36 

2.41 

5.06 

4.50 

3.39 

.15 

.70 

1.07 


51.31 49.48 
15.42 1 13.44 


COi 


0.46 

.70 

.09 

None. 


9.67 
.49 
.37 


12.11 


AltO. 


pjOft 


.53 


FeiOi 


.55 
10.78 
2.09 
2.05 
1.39 
3.63 

.17 
3.24 


.02 
11.78 
4.25 
1.66 
1.45 
1.79 

.33 
3.60 


FeSi 


2.10 


FeO 


ZnS 


1.02 


MgO 


PbS 




.55 


CaO..... 


MnO 


.12 
.10 
.05 


1.13 
.08 
.01 


.97 


NajO 


BaO 




KtO 


SrO 




HtO— 






H1O+ 


100.55 


103.45 


106.15 


TiO, 


1.07 , 1-07 : 








1 





1. Chemical analjrsls of fresh diorlte porphyry. 

2. Composition of same volume of partly altered rock in pereenta^ of original mass. 

3. Composition of same volume of completely altered rock in percentage of original mass. 

In a comparison of this kind small fluctuations, such as are shown by the phosphoric 
anhydride, are of doubtful significance and may be due to unavoidable errors in analysis and 
calculation. The important facts brought out are the notable decrease in silica, alumina, 
ferric oxide, lime, and alkalies, accompanied by an increase in ferrous iron, magnesia, and water, with 
the introduction of sulphides -of iron, zinc, and lead and of a large quantity of carbon dioxide. 

The chemical changes in the rock are graphically displayed in the accompanying diagram 
(fig. 10), which is similar to that used by Pirsson^ for illustrating magmatic differentiation. 
The abscissas here used are, from right to left, the distances from the vein of the rocks analyzed; 
the ordinates are molecular proportions of the oxides. With the object of condensing the 
diagram the siUca curve is plotted 70 ordinate divisions below its true position. The very 
great increase in carbon dioxide, water, and ferrous iron and the nearly parallel decrease in 
siUca, Ume, and soda are well brought out by the diagram. Titanic oxide, remaining constant, 
appears as a horizontal line. Both magnesia and manganous oxide show a notable increase on 
the whole. Ferric oxide decreases inversely as the increase of pyrite. Potash, after increasing 
slightly, shows a marked decrease. 

These changes may also be graphically illustrated by the distortion along radii corresponding 
to the various oxides of a circle representing fresh diorite porphyry. A diagram of this kind is 
shown in figure 11. Here the percentage of gain or loss in each oxide is plotted and the diagram 
is a graphic representation of the relations recorded in the numerical table on this page. 

MINERALOGICAL CHANGES. 

It would be of interest to compare also the mineralogical compositions of the fresh and 
altered rock, more particularly to calculate the quantity of each original mineral in a given 
volume of fresh rock and to compare that composition with the mineral .aggregates making up 
equal volumes of the altered varieties; but the presence in the fresh rock of the complex minerals 
biotite, hornblende, and diopside, the exact composition of none of which is known, makes 
this procedure impracticable. By the help of the microscope and the chemical analyses the 
following approximate mineralogical compositions are obtained for the altered rocks: 

Approximate mineralogical composition of altered diorite porphyry. 



Sericite... 

Quartz 

Kaolin! te. 
Carbonate. 

Rutile 

Apatite... 



44.8 

2S.4 



24.9 

1.0 

.5 



30.5 

31. () 

2.8 

29.3 

1.0 

1.3 



Pyrite 

Sphalerite. 
Galena 



0.4 



100.0 



2.0 

1.0 

.5 



100. 



1. Altered rock 10 feet from vein. 



2. Altered rock close to vein. 



These figures are admittedly not of high accuracy. The analysis of the rock farther from 
the vein contains nearly 3 per cent too little alumina and nearly 0.5 per cent too little carbon 
dioxide to satisfy the proportions required by the aggregation of minerals given above. If the 
analysis is correct, this fact indicates the presence of some nonaluminous silicate containing 



1 Pirsson, L. V., Igneous rocks of the Little Belt Mountains, Montana: Twentieth Ann. Rept. U. S. Geol. Survey, pt. 3, 1900, p. 571. 



100 GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

part of the alkalies here calculated as sericite; but no mineral that would account for this 
discrepancy has been recognized under the microscope. Both analyses show an excess of 
water over that required for sericite. The carbonate is highly ferruginous with only a small 
admixture of calcium carbonate. The proportions calculated for the rock close to the vein are 
as follows: 

Carbonates in altered diorite porphyry close to vein. 
Siderite (FeCOj) 63. 9 

Mflgneeite (MgCOa) 29.6 

Rhodochrosite (MnCOs) 5. 2 

Calcite (CaCOs) 1. 3 

100.0 

Microscopically the carbonate appears to be a homogeneous mixture of these isomorphous 
salts. It belongs to no well-defined mineral species but might be classed as a very impure 
siderite. The carbonate of the less-altered facies has nearly the following composition: 

Carbonates in altered diorite porphyry 10 feet from rein. 
Siderite 69. 3 

Magnesite 17.5 

Rhodochrosite 7. 3 

Calcite : 6.9 



100.0 

This agrees with the observed fact that the carbonate generally associated with the ores is 
not all identical in character. The titanium is calculated as rutile, because this mineral is 
known to occur in similarly altered rocks, although it has not been recognized microscopically 
in those here studied. 

CHARACTER OF SOLUTIONS THAT EFFECTED THE ALTERATION. 

If the alteration of the diorite porphyry was effected by the continuous action of solutions 
of one kind and origin it appears that these were rich in bicarbonates of iron, magnesium, man- 
ganese, and calcium but poor in silica and alkalies. They carried also sulphur, zinc, and lead 
with probably free carbonic and sulphydric acids. Notwithstanding the comparatively shallow 
depth at which the samples studied were collected, they do not suggest the action of water 
derived directly from the surface. The occurrence of a Uttle kaolinite close to the vein (shown 
in the calculated composition on p. 99 but not certainly identified under the microscope) may 
record slight modification by waters from the surface of rock previously altered by ascending 
vein solutions. 

ALTERATION OF THE QUAKTZ MONZONITE POBPHYBY (SELICIC TYPE). 

GENERAL CHARACTER. 

The alteration of the siUcic type of monzonite porphyry has not been as fully investigated 
as that of the diorite porphyry, chiefly because no opportunity was found of studying the change 
from fresh to altered rock at any considerable depth in any single locality. Alteration is well 
displayed in the Jessie mine, for example, but the accessible workings are all shallow and meta- 
somatism has modified all of the porphyry exposed in or near the mine so that fresh material 
for comparison is not locally available. This rock, with its large phenocrysts of feldspar, 
appears to have been more generally permeable to vein-forming solutions than was the close- 
grained diorite porphyry of the WeUington mine. Consequently extensive masses of the rock 
contain abundant disseminated pyrite, sphalerite, and galena. The feldspar phenocrysts were 
particularly susceptible to attack and the pseudomorphous replacement of these by aggregates 
of the three sulphides mentioned is generally a characteristic feature of the porphyry in the 
vicinity of the ore-bearing fissures. The microscope shows that the siliceous porphyry of the 
Jessie mine is completely altered to an aggregate of quartz, sericite, and sulphides, the original 
quartz phenocrysts being the only important primary mineral remaining. The former biotite 
phenocrysts, though retaining their shape, are. changed to sericite or muscovite and, like the 
orthoclase crystals, are in general partly replaced by crystals of pyrite, sphalerite, or galena. 



METAMORPHISM. 



101 



The general outlines of the feldspar phenocrysts are still recogmzftKie^. although the pseu- 
domorphic aggregates merge to some extent with the groundmass. •//-'.'•. 



CHEMICAL AND MINERALOGICAL CHANGES. 



A chemical analysis of the altered porphyry from the Jessie mine is gijv.en iii-Q0liimn,3 of 
the following table, analysis of similar fresh porphyry being placed under 1 and 2 for comparison: 



Chemical analyses of fresh and altered quartz monzonite porphyry. 

[R. C. Wells, analyst.] 



• • • .• 



SiOj... 
AljO... 
Fe,0^. 
FeO... 
MgO... 
CaO... 
Na,0.. 
K|0... 
H,0-. 
H,0+. 
TiO,... 
ZrO,.. 
C0|.... 



1 

1 


2 


3 

1 


67.53 


68.14 


69.61 ' 


15.46 


15.29 


15.12 


2.18 
2.42 


.35 
1.66 




............ 

a. 37 ! 


.16 


.26 


Trace. 


3.24 


3.03 


.05 


3.24 


3. 59 


.42 


3.86 


4.07 


4.54 1 


.23 


.40 


.27 1 


.55 


.39 


2,01 


.41 


.36 


.36 


.02 


.01 


.02 1 


.03 


.22 


None. ! 



P«05. 

F.... 

NiO. 

MnO. 

BaO. 

SrC. 

FeSj. 

ZnS.. 

PbS, 

CuS.. 



0.01 
.03 

None. 
.10 
.07 

None. 
.09 



99.63 



0.17 
None. 
.01 
.12 
.03 
.03 

1.52 



99.65 



0.04 



Trace. 
.14 

None. 

None. 

6.92 

.08 

.06 

.17 



100.18 



a Ferrous Iron determination uncertain in presence of sulphides. 

1. Brewery Hill. Fresh. 

2. Mouth of 13rowns Ouich. Contains considerable pyrite, but is generally fresh. 

3. Jessie mine. Much altered. 



As the fresher rocks are not from the same locality as the altered rock, and probably neither 
was quite identical with the original rock of the Jessie mass, no such detailed comparison has 
been attempted as was made for the rocks of the Wellington mine. The analyses themselves 
illustrate in a general way what has taken place. There has been a decided loss in most of the 
constituents. SiUca and alumina have not been greatly changed, and the potash may possibly 
have been actually increased. Water, sulphur, and heavy metals have certainly been added. 
The mineralogical composition of the altered porphyry may be calculated as follows: 

Approximate miruralogical composition of altered porphyry of the Jessie mine. 

Per cent 
by weight. 

Quartz 52. 

Sericite 40. 2 

Pyrite 7. 1 

Sphalerite and galena 2 

Other constituents 5 



100.0 



This same kind of alteration — pronounced sericitization with Httle or no development of 
carbonates — is found also at the I. X. L. mine, and appears to be generally a characteristic 
accompaniment of these deposits in the siUceous monzonite porphyry. Not only has no carbon 
dioxide been added to this rock, but the alkaline earths originally present have been virtually 
all removed. Within the region of ore deposition the contents of the ore-bearing solutions 
appear to have been notably different in the two types of porphyry, for the observed divergence 
in effects could not be accounted for wholly by differences in the composition of the rocks on 
the assumption that the solutions were identical at the places where the ores were precipitated. 
On the other hand, the solutions do not appear to have been radically or originally different. 
Their contents in certain constituent bases appear to be significantly related to the compositions 
of the two porphyries. 

Near the surface the rock that is altered in the manner just described has its sericite partly 
or wholly changed to kaolinite. 

PROPYUCTIZATION. 

The propylitic type of metamorphism is to some extent illustrated by nearly all of the 
monzonite porphyry. In every specimen of that rock collected from surface exposures the 
biotite shows the characteristic alteration to lenses of chlorite, with interlamellar grains or 



* • 



102 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

» 

plates of calcite or ^piStote or both. This pseudomorphous change is illustrated in Plate X, 
A and B (p. 54^.\'-TH^ pyroxene and hornblende have generally been transformed partly or 
wholly to aggi^egatesof these same secondary minerals^ and the feldspars as a rule contain some 
calcite, seneHf^', and kaolinite. There is generally, too, more or less development of quartz and 
pyrite at' tKe' expense of the original minerals of the rock, although the sulphide is not normally 
abkndjEmt. The carbonate, called calcite for convenience, is probably not all pure calcium car- 

' • * ^ 

.bpybAte, but has on the whole the habit and optical character of calcite rather than of the ferru- 
' ginous and magnesic carbonate described on page 100 as formed by the alteration of wall rock. 

The facts available in the Breckenridge district indicate that this propylitic alteration is 
restricted to rocks within 300 feet of the surface. It is characteristic, for example, of all known 
surface exposures on Mineral Hill, while long tunnels into the same hill, such as those of the 
Wellington and Rose of Breckenridge mines, reveal nearly fresh material. The rock cited on 
page 95 for comparison with altered wall rock is not propylitically altered ; neither does that kind 
of alteration appear to be a significant accompaniment of the veins or to intervene between fresh 
rock and the carbonated rock of the vein walls. Finally, the propylitic alteration does not, in 
this district, appear to be any more characteristic of those areas or rock masses containing 
ore deposits than of others in which ore-bearing solutions have apparently not been active. 

Much of the quartz monzonite porphyry has also been propylitically altered, but as it 
contains a smaller proportion of femic constituents the change is not so conspicuous as in the 
more calcic porphyries. 

That propylitic metamorphism of andesitic or dioritic rocks is a very characteristic accom- 
paniment of ore deposition in many mining regions is of course well known, and there can be 
little question that the alteration in some of those regions has been effected through the wide- 
spread penetration of the rocks by carbonate solutions, or by solutions carrying carbon dioxide 
and hydrogen sulphide, that have worked outward from the ore-bearing fissures. PropyUtiza- 
tion is undoubtedly in most cases a form of hydrothermal metamorphism, yet, as Rosenbusch ^ 
remarks, it is not to be denied that ordinary weathering might produce similar results. The 
chief agent ui the change appears to be carbon dioxide in solution, and it is reasonable to sup- 
pose that the work of dilute carbonate solutions representing the weaker and cooler phases of 
hydrothermal action may be closely imitated by that of similar solutions percolating down- 
ward from the superficial zone of oxidation and disintegration. In other words, although it 
is perhaps objectionable to speak of propyhtic alteration without qualification as weathering, 
the change may nevertheless be due to the deeper action, for the most part below water level, 
of solutions derived from the zone of ordinary superficial weathering. Some such explanation 
appears to be necessary to account for the facts observable at Breckenridge. It is possible that 
the very common association of propylitization with ore deposits may in part be assignable 
not so much to the direct action of the original ore-bearing solutions as to a difference in the 
vadose solutions later produced by weathering in metallized as contrasted with unmetallized 
districts. In metallized districts the presence of sulphides in the disintegrating and oxidizing 
rocks gives rise to solutions containing one or more of the sulphur acids and these not only 
increase the chemical activity of the vadose waters but provide them with sulphur for the 
formation of pyrite in the deeper zones at the expense of the magnetite and iron-bearing sili- 
cates in the fresh rock. This view, while connecting propyhtic metamorphism in a secondary 
or remote way with hydrothermal activity, does not require the same close relationship between 
the veins and the propyhtically altered rocks as the view postulating contemporaneity of ore 
deposition and propyhtization. Where, in connection ynth the development of chlorite, 
calcite, and epidote, pyrite also is abundantly formed or introduced there would seem to 
be good reason, in the absence of opposing evidence, for regarding the alteration as hydro- 
thermal; but where, as at Breckenridge, the modified rock is not conspicuously and generally 
pyritized the way appears to be open to an explanation involving the action of solutions 
working downward from the surface. This second hypothesis best fits the facts in the Breck- 
enridge district, so far as they can be determined with the present unsatisfactory opportunities 
for underground study. 

1 Rosenbusch, II., Mikroscoplsche Physiognphle der masslgen Gesteine, Stuttgart, 190S, p. 1105. 



CHAPTER VIII. 
THE PRIMARY ORE DEPOSITS IN GENERAL. 

MINING. 

GENERAL STATE OF ACTIVITY AND DEVELOPMENT. 

As it will be necessary in this and in succeeding chapters to refer frequently to the several 
mineS; a general preliminary account of them with particular reference to development and 
situation may appropriately be given in advance. 

In 1909 four mines only were shipping at all steadily. These were the Wellington, Country 
Boy, and Sallie Barber, in French Gulch, and the Laurium, or Blue Flag, in Illinois Gulch. 
Of these only two, the Wellington and Country Boy, can be said to have maintained an impor- 
tant production. The Hamilton and Puzzle mines were being reopened, it is true, and in a 
few other places two or three men — as a rule lessees — were at work taking out a little ore or 
prospecting. Outside of the area thoroughly studied the Arctic and Ling mines, near the head 
of the Blue, were being operated in a small way. On the whole, therefore, deep mining in this 
district is undeniably at a low ebb. 

In the early days of mining shafts were more popular than they later became and many of 
such openings sunk on the hilltops were afterward undercut by tunnels. A few shafts, among 
which may be mentioned the Oro, in French Gulch, and the Mountain Pride, in Illinois Gulch, 
were continued below water level, in situations where tunneling was not practicable; but 
these were finally abandoned. The depth of most of the old water-filled shafts is not now 
readily ascertained. The so-called Finding deep shaft, on Shock Hill, is more than 300 feet 
deep, but how much deeper could not be learned. The Ouray, Oro, Sallie Barbor, and Little 
Sallie Barber shafts are all between 300 and 400 feet deep and it is doubtful whether there is a 
500-foot shaft within the area mapped, although just outside of that area are the Wonderful 
London, near Hoosier Pa;ss, 570 feet deep, and the Warrior's Mark shaft, near Boreas, reported 
to be 700 feet deep. The Wellington and Country Boy are worked by means of adits, and 
this was the mode of development used in most of the mines productive in the past. A few 
long tunnels, notably the Lightbum, reported to be over 2,000 feet in length, and the French 
Creek, about 2,100 feet long, have been driven as prospecting ventures. 

BEVIEW OF THE NOBTHEBN AND CENTBAL PABTS OF THE ABEA. 
UP FRENCH GULCH AND OVER FARNCOMB HILL. 

A newcomer to Breckenridge may perhaps be so fortunate as to be conducted over the 
district by some one able to point out the different mines and pleased to tell their stories to an 
interested listener. By a little exercise of the imagination and the aid of the accompanying 
maps and illustrations the reader may, if he chooses, pursue a similar itinerary in shorter time, 
although with less enjoyment than if the scenes passed in reality before his eyes. 

From Breckenridge a road (see PI. I, in pocket) leads northeast over a low wooded spur 
of terrace gravels into French Gulch, where attention is likely to be drawn, first, to the old 
hydraulic workings of the Mekka placer on the right of the road. A general view of these 
workings is given in Plate XXIII, A. To the north, on Gibson Hill, appear the dumps of the 
Alice A., Bullion King, and other tunnels driven to exploit the blankets or so-called contacts 
in the Dakota beds. At the base of the hill, near Gibson Gulch, are the New York tunnel 

103 



104 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

and the Johannesburg tunnel. At the mouth of the latter, which connects with a shaft about 
350 feet higher up the hill, is a small mill. About a third of a mile farther up French Creek, 
on the left, is the Old Union mill, equipped with rolls, half a dozen Harz jigs, and 10 Wilfley 
tables. From this a tunnel about 1,600 feet long has been driven in a northeast direction 
into Prospect Hill and connects with two shafts whose collars are about 350 feet higher on the 
spur east of Prospect Gulch. (See PL XXIV, B.) On the south side of the creek, partly- 
concealed among the trees on Nigger HUl, may be seen the shaft houses of the Dunkinand 
Juniata mines, just west of Nigger Gulch. The Juniata was fonnerly productive and at one 
time a bitter legal fight was waged between the two rival companies for the ownership of the 
Juniata vein. These shafts are now abandoned, although in 1909 lessees were working in some 
tunnels on the southern part of the Dunkin ground, on the south slope of Nigger Hill. 

Thus far up French Gulch there has been no sign of recent lode-mining activity, although 
at the bend in the creek near the Old Union mill the Reliance and Reiling dredges are busily 
engaged in turning over the deep auriferous gravels, as shown in Plate XXIV, B, A short 
distance above the locaUty where the dredges are at work the Oro workings of the Wellington 
mine come into view on the left of the road (PI. XXVII, Ay p. 128). The main adit enters the 
hill near the mill and the underground workings extend northeastward under Mineral Hill for 
about half a mile and connect with another adit known as the Extenuate (or, as usuallv writ- 
ten, X. 10. U. 8) tunnel, which is about 2,000 feet farther up French Gulch and 100 feet higher 
than the adit by the mill. The disused Oro shaft is just east of the mill. In 1909 the Wel- 
lington was shipping from 30 to 35 tons of galena and sphalerite concentrates a day. Virtually 
all work is confined to the Oro and Extenuate levels. On the side of Mineral Hill, above these 
main adits, may be seen the dumps of the Siam, Orthodox, Liberty. Wellington, and other 
old tunnels of the group, ingress to all of which is now blocked. 

On the south side of the creek, nearly opposite the Wellington mine, is ttie Country Boy, 
which in 1909, after a period of idleness, was shipping zinc ore. Here there is a lower adit, 
about 50 feet above the creek and 1,100 feet long, and an upper adit, 1,700 feet long, nearly 
200 feet above the lower one. About 1,500 feet east of the Country Boy, on the same side of 
the gulch, is the Helen tunnel, which in 1908 was being run to cut the supposed northeasterly 
continuation of the Country Boy vein. It had been abandoned in 1909 and the face could no 
longer be reached. Above it on the spur just west of Australia Gulch are the older workings on 
the same group of claims. High on the spur east of Austraha Gulch and scarcely visible from 
French Gulch on account of the trees (see PI. V, B, p. 20) are the shafts of the SalUe Barber and 
Little SaUie Barber mines, the former 365 and the latter 300 feet deep. These are small pro- 
ducers of zinc ore and the workings of neither mine are extensive. Nearly 400 feet below them 
in French Gulch is the French Creek tunnel, 2,100 feet long, driven without success nearly due 
south under Bald Mountain. 

On the north side of the gulch the precipitous slope of Mineral Hill is pierced by a number 
of abandoned adits driven to cut the north-northeast veins of the Wellington zone. The lower 
and longer tunnels are those of the Rose of Breckenridge, Minnie, and Lucky mines, the last 
two being supplied with small xmlls. The Lucky adit was formerly known as the Mineral Hill 
tunnel. Above these are older tunnels, including the Cincinnati, once a producer of high- 
grade lead ore. Finally, on top of the hill are many old shafts long since allowed to fall into 
decay. The Minnie and Lucky were both productive and the latter was in 1887 the chief 
lead-silver mine in the, district. 

Leaving Mineral Hill, the road passes through Lincoln, once an active milling and placer- 
mining center but now almost deserted, and begins the ascent of Humbug and Farncomb hills. 
On the left, near the foot of the grade, is a tunnel from which issues a strong stream of ferruginous 
water. This is the McLeod tunnel, which runs north for about 1,000 feet to connect with the 
Juventa shaft, on the west slope of Humbug Hill. This shaft produced some ore in 1889, but, 
like most of the others in the district, is now abandoned. As the road nears the crest it affords 
a view down French Gulch (PI. V, B> p. 20) and to the southeast, where Mount Guyot, a mass 




A. TERRACE GRAVELS (MEKKA PLACER) ON SOUTH SIDE OF FRENCH GULCH AS SEEN FROM 
GIBSON HILL. 

In the distance is Hoosier Pass, on the right of which is seen the even east slope of the T«nmile Range. To the left 
of the pass is Red Peak, composed of the ruddy beds of the Maroon and "Wyoming" formations with some sheets 
of porphyry, all dipping to the east. The wooded slopes of Nigger Hill occupy the middle ground. The natural 
channel of French Creek is hidden by the aspens in the foreground, the deep trench visible bemg an artificial sluice 
from placers to the left. See page 71. 



'"■^%:3fc: -M 


HHHH||^^^HHp-^>— :— >=^^- 











n- BRECKENRIDGE AND THE TENMILE RANGE FROM LITTLE MOUNTAIN. 

The view Is northwest. Directly behind the town is Shock Hill, and along the west side of the Blue ms) 
light scars of hydraulic workings in the terrace graneis The lo«, rounded hill in the distance, jjst to 1 
tree, is of Dakota quartiita lapping up on the pre-Cambnan west of Braddocks. 



THE PRIMARY ORE DEPOSITS IN GENERAL. 105 

of porphyry overlying shale, rises boldly above the timbered slopes of upper French and Little 
French gulches. Almost directly underfoot, so steep is the slope, are the buildings and tunnels 
of the Wire Patch mine, on the south side of Famcomb HiU, and almost on a level with the road 
and a little southeast of it is the open stope in the Elephant ore body, now part of the Wire Patch, 
which yielded largely in the eighties. Directly in the saddle through which the road passes 
is the Ontario shaft, where gold was first found in place 30 years ago. This is probably not 
more than 150 feet deep. The Ontario ground is opened also by tunnels from both sides of the 
ridge, all abandoned and in poor condition. 

Before the descent of the north slope of Famcomb Hill is begun a few. moments may be 
spared for the fine prospect spread out to the northeast. Near at hand may be seen the dark 
shale beds of American and Georgia gulches, from which the loose material has been washed 
by the old placer miners. Beyond them is the well-timbered upper valley of the Swan. The 
view up the middle fork of that river is nearly the same as the one shown in Plate XVII, B 
(p. 76), with the crest of the Continental Divide in the distance. Twelve miles to the northeast 
may be seen the lofty pointed summits of Grays and Torreys peaks, which dominate the rugged 
country about Montezuma and Argentine. 

The road down the hill to the Swan passes over steep slopes of dark shale encumbered by 
countless small dumps, and perforated by many tunnels, most of them small and all grouped 
with little apparent system. (See PI. XXVII, B, p. 128.) The numerous small gold-bearing 
veins trend generally northwest and have no real outcrops. Many of the tunnels have been 
run by lessees. Some are crosscuts and some follow the veins. Some are short and simple; 
others crosscut from vein to vein and connect with extensive and labyrinthine workings with 
numerous levels at short intervals connected by raises. Many can no longer be entered and 
some of the most extensive are covered at the portals with waste from subsequent excavations. 
The road to American Gulch crosses the Key West, Boss, and Reveille veins and a branch, 
skirting the hill to the Fountain vein, passes over the Carpenter, Black, Gold Flake, Wheeler, 
Graton, Silver, and Bondholder veiijs of the Wapiti group. A few lessees, perhaps half a dozen 
men, were at work on these veins in 1909. Just above the point where the road turns into 
American Gulch is the portal of the Fair tunnel of the Wapiti Mining Co. This runs south- 
westward for about 1,000 feet but cuts the veins below the level at which they were productive. 
Raises, however, connect the Fair tunnel with workings above it. 

DOWN THE SWAN AND BACK TO BRECKENRIDGE BY WAY OF GOLD RUN AND GIBSON UlLL. 

The road now continues down American Gulch to the Swan, past the mouth of Georgia 
Gulch, where a few timbers projecting from placer debris mark the site where once stood the 
placer town of Parkville; past Snider's camp, where an attempt is being made to work the 
deep gravels; and into the broader valley of the main river near the abandoned settlement of 
Swan City. Just before Swan City is reached attention is attracted by the ruined mill of the 
I. X. L. mine on the south side of the valley. This mine, developed by tunnels, produced in a 
small way for many years. Half a mile south of Swan City, on the east side of Browns Gulch, 
is the Cashier mine, having adits on three levels and some very large stopes in porphyry. For 
a number of years this yielded a low-grade gold-silver ore which was treated in a 40-stamp 
mill, now removed. 

A mile west of Swan City is Summit Gulch, with the Hamilton mine near its head, a mile 
from the Swan. This mine was worked in a primitive way for many years and then lay idle 
for a decade, until 1909, when it passed into new hands. Large stopes have been opened in 
metaUzed fissure zones in porphyry and the mine is credited with an output of about $400,000. 
Most of the ore, which was valuable chiefly for gold and silver, was concentrated in a little 
10-stamp mill. 

There are no more mines of consequence along the Swan, and unless tlie traveler wishes to 
visit the dredges near Valdoro he may advantageously turn his route across Delaware Flats 
and up through the placers of Gold Run, shown in Plate XVI, A (p, 74). Near the upper 



106 GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

end of these placers, on the northeast side of the run, is the Jessie mine, of which a general 
view is given in Plate XXIX, A (p. 144). This mine has been a fairly large producer of low-grade 
gold-silver ore similar in its occurrence to that of the Cashier and Hamilton mines. The mine 
is extensively developed by four adits, of which parts of two only, the Glenwood and Jessie, are 
now open. These are approximately on the same level and connect. The ore mined probably 
averaged from $3 to $6 a ton, and the total production is variously reported at $800,000 to 
$1,500,000. There is a 40-stamp mill at the mine. 

Leaving the Jessie, the road ascends the north slope of Gibson Hill and passes successively 
the Extension, Jumbo, and Little Corporal mines, all abandoned and inaccessible. The Jumbo 
produced considerable gold ore from the oxidized portion of a vein in monzonite porphyry and 
quartzite, most of which was milled at the mouth of Cucumber Gulch. The Extension com- 
pany worked what is supposed to be the same vein and milled the ore in a 20-stamp mill at 
the mine. Both mines were opened by adits and did not attain great depth. Of the Little 
Corporal, worked through a shaft in quartzite, no details could be ascertained. 

Crossing over the crest of Gibson Hill through the pass near the Little Corporal, the road 
skirts the west side of the hill and affords a broad outlook over the valley of the Blue. (See 
PI. XIV, J?, p. 70.) Below the road, along the west base of the hill, are the Sultana, Fox Lake, 
and other tunnels on two or more pyritized beds in the strip of "Wyoming" formation exposed 
along the Blue. The two named have produced some ore — ^how much is not known, although 
the Sultana shipped both carbonate and sulphide ore with considerable regularity from 1888 
to 1898. Higher up the hill, near the road, are the Eureka shaft and the Kellogg and Alice A. 
tunnels, all exploiting flat-lying metallized beds in the Dakota formation. The Kellogg is 
credited with a considerable production and was one of the well-known mines in the early 
eighties ; but Uttle reliable information can be obtained regarding the workings and the char- 
acter of these deposits can be learned imperfectly only through hearsay. Across the Blue, in 
Shock Hill, may be seen the Iron Mask mine (PI. XIV, jB, p. 70), which was for many years a 
producer of rich lead-silver ore from " contacts '* above and below a sheet of porphyry, shipping 
for a time at the rate of two or three cars a week. It was worked through an 1,800-foot adit. 
The total output is said to have been about $50,000. On the top of Shock Hill is the Finding 
shaft, recording an apparently ill-advised attempt, dating from about 1898, to find ore bodies 
resembling those of Leadville. On the west side of the hill, not visible from Gibson Hill, are 
the Brooks-Snider workings and dismantled 20-stamp miU. The material milled apparently 
was fractured quartzite carrying native gold. There is no vein in sight and the operations 
were certainly not profitable. From the Alice A. the road, from which was taken the view 
shown in Plate IV, fi (p. 18), descends to Prospect Gulch and French Creek. 

BE VIEW OF THE SOT7THEBN MINES. 
ILLINOIS GULCH. 

A second and shorter excursion will suffice to review the mines in the vicinity of Illinois 
Gulch, the road to which passes southward out of Breckenridge past the abandoned placer pit 
of the Gold Pan Co. On the south side of IlUnois Gulch near its mouth is Little Mountain, a 
hill of gently dipping Dakota quartzite. In the fractures of this rock have been found some 
bunches of rich oxidized gold-silver ore. In all, ore to the value of probably $40,000 to $50,000 
has been taken from this hill, mostly between 1898 and 1900; one small pocket alone, close to 
the surface above the Germania tunnel, yielded $30,000. Just east of Little Mountain is the 
portal of the Willard tunnel of the Puzzle mine. This adit, after following the Puzzle-Ouray 
vein through the hill to the Puzzle-Extension shaft in Illinois Gulch, just above the mouth of 
Dry Gulch, crosscuts north 300 feet to the nearly parallel Gold Dust vein. The Ouray workings 
adjoin those of the Puzzle on the southwest. From 1885 to 1897 these three closely connected 
mines produced much rich silver-lead ore carrying considerable gold; but the Puzzle and Ouray 
Cos., owning intersecting claims on the same vein, were for much- of this time engaged in 



J. S. OEOUDOICAL 




A. DREDGE ON THE BLUE, NEAR THE MOUTH OF THE SWAN. 

909 this boat was working m difficult ground with many huge bowlders. It wa$ not put into c< 
It has 9^ ^ot opan-connectad buckats. See page iSo, 



.^^^ 








mmm^m 


^^^^^^ 


I 

9 






-.-f^ 


-♦.* 


"iS 


' _M- i_i *-^^^ 




ir  




^-^ 




/ 





B. VIEW UP FRENCH GULCH FROM NIGGER HILL, SHOWING MINES AND DREDGING OPERATIONS. 

I. Reilirg dredge. This has just turned after a successful run upstream, i. Reliance dredge. 3. Wellington mine 



.1 (among trees), ,0. Lucky t. 






THE PRIMARY ORE DEPOSITS IN GENERAL. 107 

litigation. After long idleness the Willard tunnel was cleaned out in 1909, some ore was milled 
from the Gold Dust vein, and preparations were made to prospect the vein at depth. On the 
south slope of Nigger Hill, north of the Puzzle and Gold Dust workings, are a number of tun- 
nels on the ground of the Dunkin Mining Co., where three veins are recognized, all trending 
northeast. From northwest to southeast these are the Juniata, Dunkin, and Maxwell veins. 
Ore amounting in value tp over $65,000 is said to have been taken by lessees from the Gallagher 
tunnel on the Dunkin vein. The total production from all workings was probably much 
greater. 

About 1,000 feet farther up Illinois Gulch than the Puzzle Extension shaft is the main adit 
of the Washington mine. The Washington vein, said to be accompanied by two others nearly 
parallel, the Emmet and Atlantic veins, strikes northeastward over the spur connecting Bald 
Mountain and Nigger Hill. There are a number of tunnels on the different veins, but none could 
be entered in 1909. The mine is an old one and was first worked through a shaft on top of the 
ridge. It is said to have produced from $400,000 to $500,000, partly from shipping ore and 
partly from material treated in a 20-stamp mill. 

In the upper part of Illinois Gulch, where it widens oat into the slopes of Bald Moimtain, is 
the Laurium mine, in pre-Cambrian rocks, recently opened by the Blue Flag Mining Co. after a 
long period of idleness. This, one of the first mines worked in the district, originally produced 
lead ore, but now has a 60-ton mill and yields a pyritic concentrate carrying gold and sUver. It 
is opened by an adit and attains no great depth. It is credited with* a production of about 
$80,000. A little farther up the gulch, in the ''Wyoming" beds, is the Mountain Pride mine. 
This was developed by two shafts 500 feet apart and apparently from 250 to 300 feet deep. 
Work on this mine appears to have begun about 1888; some carbonate ore was shipped in 
1896; and in 1898 the Mountain Pride led the district in production. At this time a diamond- 
drill hole was started with the intention of going 1,000 feet. It evidently entered the pre- 
Cambrian far short of that depth, but aU efforts to obtain data regarding this drill hole, reported 
to be 1,100 feet deep, have been futile. The mine and a mill equipped with two sets of rolls, 
two 3-compartment jigs, and five Wilfley tables have been abandoned to ruin. 

Northeast of the Mountain Pride mine are the Golden Edge and the Gold Bell mines, on 
fissures in the Dakota quartzite, which extends under the porphyry of Bald Moimtain. The 
Golden Edge is abandoned, but the Gold Bell is a new prospect in which a little ore was visible 
in 1909. High on the slope above the Golden Edge is the Carbonate, a small mine in the 
porphyry with a record of a few shipments of excellent lead carbonate ore a few years ago. It 
is now idle. 

VALLEY OF THE BLUE SOUTH OF BRECKENHIDGE. 

There are still a few other mines which, although outside of the area mapped, are, broadly 
speaking, in the Breckenridge district and deserve mention here. 

About 7 miles up the Blue from Breckenridge is the Governor mine, in beds apparently 
.belonging to the ' 'Weber formation.'' This was formerly productive, but has been idle for many 
years and can not now be entered. It was worked through adits and the ore was treated in a 
small mill. Near the head of the Blue, at the base of Quandary Mountain, is the Monte Cristo 
mine, which was briefly described by Emmons* many years ago. Theore, consisting of metallized 
Paleozoic limestone, was quarried and concentrated. The mine has long been idle. A little 
farther up the Blue are the Senator, Arctic, and Ling mines, aU working through tunnels in a 
small way on gold-bearing veins in the pre-Cambrian. 

CONCLtJSION. 

This completes the general survey of lode-mining activity in the Breckenridge district. 
With the possible exception of the Wellington there is not a mine among all those mentioned 
that is systematically and extensively developed and that can be regarded as steadily and 

i Mon. U. 8. G«ol. Survey, vol. 12, 1886, p. 630, 



108 



GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 



profitably productive. By far the greater number are abandoned, with not even a watchman in 
charge. Detailed and accurate mine maps are as a rule unobtainable. Even the Wellington 
Mines Ck). keeps neither stope maps nor sections and has very imperfect records of its old 
workings. The maps of many of the abandoned mines have been carried out of the district and 
are stored away with old books and papers in different parts of the country. As ingress to the 
workings themselves is generally impracticable the data available for a^gtudy of the ore deposits 
are exceptionally meager, in spite of the hearty wiUingness of most of the men now in the district 
to further such an investigation. 

TENOR AND TREATMENT OF THE ORES. 

• 

The Breckenridge district has yielded ores of widely varied kinds and richness, ranging from 
the masses of native gold from Famcomb Hill, on the one hand, to the zinc ores of French Gulch 
or to the concentrating ore of the Jessie mine, on the other. 

The average value of the shipping ore and concentrates, exclusive of course of the Famcomb 
Hill pockets, may be roundly estimated at $25 a ton, gross. Some idea of the character of the 
ores and concentrates shipped from the district in the past may be had from the following table. 
Some of the compositions given are actual sampler analyses chosen as fairiy representative of the 
ore from a certain deposit; others are generaUzed averages of a number of analyses. 

Composition of Breckenridge ores. 



Wellington. 



Kind of material a 

Gold ounces per ton 

Silver do.. 

Lead per cent 

Zinc do.. 

Iron do. . 

Silica do.. 

Date 



Country 
Boy. 



O 

0.02 

10.00 

50 

5 

14 

5 

1909 



S 

ao2 
laoo 

45 
6 

10 

7 

1909 



6S 

0.10 

25.00 

45 



6 

12 

1909 



8 



43 
1909 



Puzzle. 



S 

0.07 

24.00 

«25 

8 

14 

24 



Gold ust 



S 

0.25 

20.00 

55 

6 

4 



Cincinnati. 



Minnie. 



cC 

0.01 

10.50 

43.5 



C 

0.03 

l&OO 

67 



10 

12 

1901 



7 
3 



Traax. 



dS 

0.06 

17.00 

20 



42 



Old Union. 



Kind of material a 

Gold ounces per ton. . 

Silver do. . . , 

Lead percent.. 

Zinc do ... . 

Iron do — 

Silica do. . . . 

Date 



S 

0.07 

S.00 

31 



Dunkin. 



8 

0.60 

15.00 

40 



12 

29 

1902 



8 
32 



Washing- 
ton. 



Iron Mask. 



Brooks- 
Snider. 



Mountain Pride. 



/S 

0.60 

26.40 

39 



7 
25 



OS 

0.03 

21.00 

58 



C(?) 
1.3 
3.0 



3 

9 

1901 



3 

71 
1898 



Kellogg. 



S 


c 


0.3 


1.2 


9.0 


8.5 


50 


30 


7 


12 


9 


15 


5.5 


6 


1902 









0.06 

20.00 

59 



4 

9 

1899 



Kind of material a 

Gold ounces per ton. 

Silver do. . . 

Lead percent. 

Zinc do... 

Iron do. . . 

SUica do... 

Date 



Cashier. 



oQ 

0.55 

10.00 



5 

40 

5 

1904 



Wire 
Patch. 



C 

0.90 
10.00 



34 

10 

1908 



Jessie. 



C 

1.07 

11.89 

6 

13 

30 

5 

1897 



Hamilton. 



6 
0.67 
4.15 



39 

6 

1903 



Gennania. 



8 
0.12 
70.00 
U) 



S 

0.17 

152. 61 

8.7 



80 



3 

78 



Carbonate 

(Bald 
Mountain) 



*S 

0.4 

25.0 

40 



12 
5 



Etigrade 

(Little 

Mountain) 



sS 

0. 20-1. 65 

500-726 

4-13 



9 
66 



Jumbo. 



C 

0.46 
L90 



35 

16 



Boss. 



<S 
36 
27 



190& 



a S, Shipping ore; C, concentrates. 

b Carbonate ore from surface workings; 1.5 per cent of sulphur. 

c 5 per cent of sulphur. 

d Partly oxidized. 

« Below average. 

/ Horn tunnel; 5 per cent of su phur. 

9 Oxidized ore. 

A Oxidized ore; 1.5 per cent of sulphur. 

i Tailings from a rich pocket. 

J Otherwise similar ore carries up to 30 per cent of lead. 



THE PRIMARY ORE DEPOSITS IN GENERAL. 109 

Millin g has been practiced in the district from an eariy period in its history. One of the 
first mills was built at Lincoln to treat ore from the Old ReUable vein and was run by water 
power* Another eariy mill was the 20-stamp Brooks-Snider, on Shock Hill, erected in 1881, 
but never thoroughly successful. In January, 1886, the local milling faciUties were as follows: 

Eureka mill, Cucumber Gulch .• 30 stampe. 

Washington mill, Bald Mountain .* 20 stampe. 

Brooks-Snider mill, Shock Hill 20 stamps. 

Lincoln mill, Lincoln 15 stamps. 

Wheeler mill, Lincoln 10 stamps. 

Blanchard mill, Lincoln 5 stamps. 

Ryan & Hartman mill, Lincoln Huntingtons. 

Russell & Nashold mill, Breckenridge Wiswell pulverizers. 

At tliis time also a 10-stamp mill was under construction where the present Jessie mill 
stands and there was a mill at the Laurium mine, then idle. 

In 1909 the only mills in fairly steady operation were the Wellington mill, equipped with 
rolls, jigs, and tables, and having a capacity of about 100 tons a day, and the Laurium 60-ton 
mill, with pulverizers and tables. For a part of the year the old Ware mill, west of Breckenridge, 
originally built in 1889 to treat ore from Farncomb Hill, was run on ore from the Gold Dust 
vein, and the old 10-stamp mill at the Hamilton mine was started late in the summer. Small 
Tnilla were being prepared for work also at the Gold Bell mine, on Bald Mountain, and at the 
Arctic mine, up the Blue. Standing idle, in various stages of dilapidation, but none quite in 
ruins, were the Wire Patch concentrating mill, which was in operation in 1908; the Old Union 
mill, with rolls, jigs, and 10 concentrating tables; the Extension 20-stamp mill; the Jessie 
40-stamp mill; the Wasliington 20-stamp mill; and half a dozen smaller ones. The Cashier 
40-stamp mill has been wholly dismantled. 

Various attempts at smelting in the district were made prior to 1890, but of course were 
unsuccessful. The brick stack of the Wilson smelter, built in 1880 and afterwards converted 
into the Breckenridge 20-stamp mill, marks the site of one of these enterprises just north of 
town. The mill has long since disappeared. 

The marketing of ores and concentrates was facilitated by the sampler built at Breckenridge 
in 1897 by the Kilton Gold Reduction Co. These works subsequently passed into the hands 
of the Cliamberlain-Dillingham Ore Co., which is closely connected with the American Smelting 
& Refining Co. 

The freight rates on ores from Breckenridge to Colorado smelters ranged in 1909 from $1.50 
to $3 a ton on ores up to $18 a ton in assay value, with no increase on higher grade ores. 

On crude sulphide lead ores the treatment charges by the smelters range through a sliding 
scale from $6.50 a ton for 5 per cent ore to 50 cents a ton for 40 per cent ore. No smelting 
charge is made for sulphide ore carrying over 40 per cent of lead. The prices paid per unit of 
lead* range similarly from 30 cents on 5 per cent ore to 42 cents for 40 per cent ore. For higher- 
grade ores the price ranges from 42 to 46 cents. These prices are on a so-called neutral basis 
(that is, an assumed quotation) of $4 a hundredweight for lead. They are increased or dimin- 
ished by 1 cent for every 5 cents by which the quotation used as a basis for settlement exceeds 
or falls short, respectively, of the neutral basis. This quotation is not the actual New York 
quotation, but is 90 per cent of that figure, provided the latter is not over $4. Wlien the New 
York sales price exceeds $4 the smelter-settlement quotation is $3.60 plus one-half of the excess. 
Thus the New York sales price will have to be $4.80 in order that the sliipper shall receive for 
his lead the actual price per unit on the $4 basis as given in the smelter schedule. Excess of 
zinc over 10 per cent is penalized at 50 cents a unit and excesses of iron and silica at 10 cents a 
unit. Gold, when under 3 ounces a ton, is paid for at $19 an ounce; when over 3 ounces, at 
$19.50 an ounce. Silver is settled for at 95 per cent of the current New York quotation. 

1 The " unit" Is 1 per cent of lead per ton of ore, or 20 pounds. 



110 



GEOLOGY AND OBE DEPOSITS OF BBECKENBroGE DISTRICT, OOLOBADO. 



On lead concentrates carrying from 5 to 30 per cent of lead the treatment charges range 
from $3.75 to $2.25 a ton. Concentrates with over 30 per cent of lead take the sulphide-ore 
schedule. The sdnc limit in concentrates is 5 per cent, with 30 cents a unit penalty for excess. 

Carbonate lead ores are subject to the following schedule: 

Prices paid and treatment charges for carbonate lead ares. 



Lead contents. 



Under 5 per cent 

From 5 to 10 per cent, inclustve. 
Over 10 to 15 per cent, inclusive 
Over 15 to 20 per cent, inclusive 
Over 20 to 25 per cent, Inclusive 
Over 25 to 30 per cent, Inclusive 

Over 30 to 35 per cent, inclusive 
Over 35 to 40 per cent, inclusive 
Over 40 to 45 per cent. Inclusive 
Over 45 to 50 per cent, inclusive 
Over 60 per cent 



Price per 
unit 


Treatment 


charge per 


(cents). 


ton. 


$4 basis. 







15.00 


35 


4.50 


38 


3.00 


40 


2.00 


42 


1.00 


44 


.00 


$4.50 basis. 




46 


.00 


48 


.00 


50 


.00 


51 


.00 


52 


.00 



Maximum 

treatment 

charge per 

ton. 



t9.00 
8.00 
7.00 
6.00 
4.50 
3.00 

1.50 
.50 
.00 
.00 
.00 



On these ores 25 cents a unit is charged for sulphur in excess of 3 per cent up to the maxi- 
mum of the sulphide-ore schedule. The zinc limit is 8 per cent, with a penalty of 50 cents a 
unit for excess. 

For siliceous ores containing over 65 per cent of insoluble silica and ranging from $14 to 
$100 a ton in assay value the charges are from $5 to $13 a ton. When the siUca is imder 65 
per cent and the gross value under $50 a ton, the working charges are from $6 to $10 a ton. 
In this group the richer the ore the higher the treatment charge. 

The zinc ores of the district do not pass through the local sampler. Ores and concentrates 
containing over 30 per cent of zinc and generally less than 10 per cent of iron go to the smelters 
of Missouri and Kansas. The zinc in the ore, less 8 units, is commonly paid for by these 
smelters at the current St. Louis price for spelter less from $14to$15a ton of ore, according 
to its quaUty. This, with spelter at 5^ cents a pound, would yield about $20 a ton on 40 per 
cent ore or $18 a ton on 38 per cent ore. The sphalerite middlings from the Wellington mill 
go to the Western Chemical Manufacturing Co., in Denver, which pays for the sdnc and after 
separating the sphalerite utilizes the pyrite in acid making. 

GENERAL. CHARACTER OF THE FISSURING. 

The district is far from being a favorable one for the study of fissure systems, owing to the 
soil-covered condition of the slopes and to the fact that the veins, having very little quartz in 
their composition, do not outcrop visibly and can not be traced by the eye over the surface. 
Moreover, as stated in the first part of this chapter, the conditions for undergroimd study are 
fully as unsatisfactory as at the surface. 

With no known exception, certainly with no important exception, the veins strike from 
east-northeast to northeast, showing in this respect rather remarkable uniformity. They thus 
cross nearly at right angles the general strike of the sedimentary formations. As Spurr and 
Garrey* have pointed out, this is the prevailing direction of the principal fissuring throughout 
the great Colorado mineral belt extending from Boulder County to the San Juan region. 

The dips are more varied than the strikes and range generally from 45° to 90°. As a 
whole, the veins stand at fairly high angles and the average dip is probably near 65°. The 
important veins of the Wellington group and of Mineral Hill generally dip at angles ranging 
from 45° SE. to 90°. The usual dip is about 65°. The Minnie vein, where seen, dips 70° SE. 
The Old Reliable vein, wliere cut by the McLeod tunnel, near Lincoln, dips 75° SE. On the 
opposite side of French Gulch the Coimtry Boy and Sallie Barber veins dip northeast at about 
80°, while a vein cut near the face of the Helen tunnel and at least one small vein in the Country 

» SpuiT, J. E., and Oarrey, G. H., Economic geology of the Georgetown quadrangle, Colorado: Prof. Paper U. S. Geol. Survey No. 03, 1908, p. 118. 



THE PRIMABY ORE DEPOSITS IN GENERAL.. Ill 

Boy workings dip southeast at about 65*^. The little veins of Famcomb Hill are generally 
steep. Most of those in the western part of the hill in the Ontario, Key West, and Boss mines 
dip northwest at 60® to 75®. Those in the eastern part of the hill, on the other hand, generally 
dip southeast at about the same angles. A few are practically vertical. In the Hamilton and 
Jessie mines the fissuring is complex and there are many subordinate intersecting systems. 
The dip of the dominant fissures, however, is northwest and the angles range from 45° to 
vertical. The Gold Dust and Puzzle veins are so nearly vertical that their general inclination 
was not determinable from the Uttle that could be seen of them in 1909. The veins of the 
Washington mine are reported to dip to the northwest at 65® to 75®. The general dip of the 
Dunkin vein is southeast at 50®. The Laurium vein dips 70® in the same direction. 

The principal fissures in the district are thus referable to a single conjugate system with 
strikes which range from east to north, but which, as a rule, are between east-northeast and 
northeast and with generally steep northwest or southeast dips. 

The fissiu*es are not of great length. The Wellington group of fissures has a total length 
of at least IJ miles. The Siam vein of this group is probably the largest lode in the district, 
but has not been proved to have a length of more than 1,200 feet, to which probably 400 feet 
more should be added for its faulted continuation — the Iron vein. The Dunkin veih has been 
opened continuously for 1,400 feet, the Puzzle vein for about 1,700 feet, and the Washington 
vein for fully 1,200 feet. The other veins known in the district are probably all shorter, so far 
as exploration has shown, than the ones mentioned. As they are not of great length, the veins 
evidently do not fill structurally important fault fissures — a conclusion amply borne out by 
their geologic relations and by the general absence of much triturated material between their 
walls. Concerning the behavior of the fissures at considerable depth, the imdergroimd workings 
as yet afford no data. 

CLASSIFICATION AND DISTRIBUTION OF THE DEPOSITS. 

In accordance with the usage of Beck ^ the word prunary, as used in the heading of this 
chapter, implies essentially nothing more than a distinction from detrital or placer deposits. 
In this sense even such veins as may have undergone some so-called secondary enrichment in 
their upper parts are primary deposits as a whole. 

To one having only a slight familiarity with the district the primary deposits might appear 
obviously susceptible of division into definite groups. Nothing, for example, could at first 
glance appear more different than the narrow gold-bearing veins of Famcomb Hill, generally 
less than an inch wide, and the great galena-sphalerite-pyrite veins of the Wellington mine, in 
French Gulch, or the low-grade stockwork of the Jessie mine, in Gold Run. Fuller knowledge, 
however, shows that the differences are largely matters of mere size and shape of relative propor- 
tions of constituents rather than of essentially distinct types or periods of mineraUzation, and 
that deposits apparently of wide diversity are connected by those of intermediate character. 
Thus there is a close relationship, although not much similarity, between the gold-bearing 
Famcomb Hill veins, in shale, and the Elephant ore body of the Wire Patch mine, on the south 
side of the hill, in porphyry. The latter in turn is in some respects not unlike the ore bodies of 
the Cashier mine, in Browns Gulch, and of the Jessie mine. Finally, the ore bodies of the 
Hamilton mine, in Summit Gulch, partly bridge the gap between the deposits of the Jessie and 
Wellington mines. Accordingly, any grouping artificially made must depend more on differences 
in the shapes of the ore bodies or in the relative proportions of the constituents than on essential 
differences in genesis. With this understanding the ore deposits of the Breckenridge district 
may be conveniently divided into (1) veins of the zinc-lead-silver-gold series; (2) stockworks 
and veins of the gold-silver-lead series; (3) the Famcomb Hill gold veins; (4) veins in the 
pre-Cambrian rocks; (5) metasomatic replacements, generally along the bedding planes, in 
sedimentary rocks; and (6) gold-silver deposits in the Dakota quartzite. 

The deposits of the first class occur chiefly within the south half of the area mapped and 
are in their distribution closely related to the monzonite porphyry. Many are in this rock; 

1 Beck, Richard, Lehre von den Enlagerst&tten, 3d ed., Berlin, 1900. 



112 



GEOLOQY AND QBE DEPOSITS OF BEECKENBIDGE DISTBICT, COLOEADO. 



others have the porphyry for one wall or are partly m porphyry and partly in the Dakota quartz- 
ite. In this group belong the ore bodies of the Wellington, Lucky, Minnie, Cincinnati, and 
other mines on the south slopes of Prospect and Mineral hills ; of the Country Boy, Sallie Barber, 
and Little Sallie Barber mines, on the slope of Bald Moimtain south of French Creek; of the 
Washington, Dunkin, and Juniata mines, on Nigger Hill; and of the Puzzle, Ouray, and Gold 
Dust mines, in the vicinity of Illinois Gulch. The formerly productive ore body of the Mountain 
Pride mine, near the head of Illinois Gulch, should probably be considered in this group, although 
it occurs in an area of sedimentary rocks ; for the dumps show that there was monzonite porphyry 
associated with the ore. 

In the second group are the ore bodies of the Jessie, Hamilton, Cashier, I. X. L., and Wire 
Patch mines. In the third group are the unique veins that traverse the north slope of Famcomb 
Hill. The fourth group includes the Laurium, Senator, Arctic, and ling mines. In the fifth 
group are the Kellogg, Bullion King, Sultan, and other mines on Gibson Hill, and probablyalso, 
the Iron Mask mine. Finally, in the sixth group, not a very well defined one, are the Brooks- 
Snider and some other workings in Shock Hill, the ore pockets of Little Mountain, and possibly 
also the Golden Edge and Gold Bell mines, on BaM Mountain. 

The meager facts ascertainable regarding the Jumbo, Extension, and Little Corporal mines 
are included in Chapter IX, on the veins of the zinc-lead-silver-gold series, although too little 
is known of these deposits to warrant their definite assignment to this series. 



INDEX TO MINING CLAIMS. 



The topographic map (PI. II, in pocket) shows the location of the numerous claims in the 
district by a system of numbers. The district is divided into squares, which are indicated by 
letters and roman numerals on the mai^ns, and the claims in each square are numbered, as 
indicated in the following lists : 



Alphabetic list of claims shown in Plate II. 



A VIC 32 

Abbot Placer V B 9 

Abby VF9 

Ada Placer VI A 1 

Ada Placer V C 63 

Adams Placer IV A 2 

Adelaide IV D 62 

Aggie VIC 22 

Aggie IVD41 

Afax VII E 29 

A.Jay IVD15 

Alameda VI D 16 

Albany III D 48 

Albany V F 42 

Alice A. IV B 62 

Alice IVB67 

Alice V1D55 

Alice A. (M. S.) V B 19 

Alice and Nellie Placer III F 38 

Alliance IV B 51 

Almira III E 2 

Alpha Placer VI D 60 

Alta VIE 6 

American III D 82 

American II F 2 

American Placer IV A 1 

American Placer (Survey 84) V G 2 

American Placer (Survey 85) V G 1 

Amulet III F 23 

Andromeda V D 57 

Anna IV B 47 

Anna VI C 51 

Ann Arbor VI D 36 

Ann Arbor Placer VI D 35 

Annex Placer VII D 7 

Annie B. IV F 8 

Annie C. Ill C 36 

Annie Placer III A 7 



ANo. 1 VE25 

Antelope III E 36 

Apex V A 27 

Arab III C 20 

Ardath IV D 49 

Arling IIIC4 

Arraria V F 21 

Arthur IV D 2 

Arthur Nail VI E 2 

Athol VC12 

Athofl IVB66 

Atlantic VI C 46 

Auckland V D 39 

Australia Gulch Placer V D 68 

Autocrat VI B 11 

Autocrat Ex. VI B 12 

Aye Kay IV F 2 * 

B VIC 33 

Bacon IV B 41 

Baden Baden III C 17 

Badger VI C 2 

Badger III D 25 

Bagdad III C 21 

Baker VII E 11 

Baldwin IV E 6 

Baldwin M. S. IV E 5 

Ballarat Placer II E 4 

Ballard VI C 5 

B. and L. No. 1 Placer II A 5 

B. and L. No. 2 Placer IV A 3 

Barbara IV B 45 

Bartlett-Shock Placer VB6 

B. B. IV C 19 

Bear VD2 

Bear Extension V D 1 

Bed Rock IVB21 

Bed Rock Placer VI G 30 



Bed Rock Placer II E 1 

Belcher VI D 18 

Belle III C 40 

Belle VF50 

Belle of Baloy VI E 17 

Belle of Swan III E 70 

Bellevue VI D 29 

Ben Butler V E 14 

Ben Harrison VI C 20 

Berlin III C 14 

Berlin Placer VI C 45 

Berten III E 17 

Bertha D. Ill C 33 

Bertie H. VI C 25 

Bess VF3 

Bessie IV C 9 

Beula IV A 17 

Big Pan VID66 

Big Sallie Barber VI D 39 

Big Sallie Barber Ex. V D 74 

Bill VE12 

Billie Button IV C 41 

Bi-metallic III D 33 

Birdie Treble VII C 5 

Birdseye III E 16 

Birdseye No. 1 III E 15 

Birdseye No. 2 III E 14 

BirdBeve No. 3 III D 68 

Black bear V E 7 

Black Prince VI E 23 

Black Prince No. 2 VI E 29 

Blackhawk VI E 44 

Blaine IV D 1 

Blaine III E 43 

Blaine VI E 39 

Blanche E. IV C 42 

Blind Tom IV F 5 

Blue IVC44 



THE PRIMABY ORE DEPOSITS IN GENERAL. 



113 



Blue Bird III D 52 
Blue Bird III E 24 
Blue River IV B 79 
Blue River V B 13 
Blue River No. 2 V B 14 
Blue River Placer II A 4 
Blue River Placer V B 11 
Bonanza V A 16 
Bonanza VII G 2 
Bonanza Placer V B 10 
Bonnie Nelson III B 2 
Boom Placer V A 1 
Bo68 VC48 
Boss VF33 
Boston III E 23 
Boy IVD50 
Braddock Placer II B 1 
Breckenridge IV B 15 
Breckenridge IV D 11 
Brewery Placer III F 42 
Bright Hope VI 8 
Brisbane IV D 59 
British Boy III F 32 
Brooklyn IV D 26 
Brooks-Snider M. S. V A 11 
Brown V C 43 
Brown VI D 37 
Brown Bear VE6 
Brown M. S. V D 24 
Brown Placer III F 22 
Brownie Birdie V D 26 
Bryan III D 45 
Bryan IV D 7 
Bryan Placer II B 4 
Buckeye V D 55 
Buffalo IV 3 
Buffalo IVD40 
Buffalo VE33 
Buffalo Placer IV 39 
Bull Dog IV A 10 
Bullion III 019 
Bullion King IV B 56 
Rulwer VI 50 
Bunker Hill VI D 7 
Burlington III D 62 
Bumsides IV B 40 
Butte City V E 36 
B. V. VIE 38 

VI 34 
Cache III E 10 
Cadiz VID19 
California V A 20 
California VI D 17 
Caliph III 23 
Camp Bird III D 24 
Canfield IV B 64 
Capt. Ryan Placer VI F 1 
Carbonate IV D 20 
Carbonate VII E 24 
Carbonate No. 2 VI E 61 
Caribou HIE 65 
Carpenter Placer IV 37 
Carpenter Placer VI 82 
Carrie La Salle V D 22 
Carvel IV D 53 
Cashier III F 13 
Cashier M. S. Ill F 11 
Cassiopeia V D 48 
Castor HIE 58 
Cecil VI El 
Cecil 0. Ill 31 
Celtic VE16 
Central American II F 3 
0. B. and Q. Ill 6 
Chance IV C 32 
Chantilly IV B 60 
Charity VD4 
CharUe V 44 

90047°— No. 75—11 8 



Charlie W. Ill E 28 

Chesapeake IV A 21 

Chester IV 50 

Chester III E 20 

Chicago III 41 

Chicago V015 

Chicago VI 6 

Chicago VII D 9 

Chief VC58 

Chippewa VI 9 

Christina V E 19 

Cincinnati V D 49 

Clara III E 3 

Clara Belle VI E 7 

Clara L. Ill 35 

Clara W. V E 39 

Clark Placer VF46 

Clay HIE 45 

Cleopatra IV D 23 

Cleopatra No. 1 IV D 22 

Cleopatra No. 2 IV D 24 

Cleveland IV B 17 

Cleveland IV 7 

Cleveland IV D 4 

Cleveland Placer IV B 28 

Cleveland Placer V B 15 

Cliff VI 58 

Clifton VA19 

Clifton III E 12 

Clifton M. S. Ill E 13 

Climax IV D 45 

Climax V F 55 

Climax No. 2 III E 73 

Climax No. 3 HI E 74 

Climax No. 4 III E 72 

Climax Placer III A 4 

Clipper III D 70 

Cloud III E 31 

Cobb and Ebert Placer V E 30 

Coin HID 31 

Colorado V A 24 

Colorado V 50 

Columbia VB3 

Columbia HI C 30 

Combination VI B 15 

Comet HI E 53 

Comet V F 6 

Comet VF23 

Comet No. 1 V F 4 

Comet No. 2 V E 29 

Comet No. 3 V F 11 

Comet No. 4 V F 12 

Commodore Placer VI G 14 

Como IVB16 

Company IV B 59 

Company No. 2 IV B 86 

Compromise VI 71 

Conclave V F 20 

Confidence VI 69 

Conjecture VI 70 

Conquest VI 68 

Conrad VI B 17 

Consolidation IV 25 

Contact VI B 20 

Convent VII F 1 

Cora E. HI C 32 

Corkscrew Placer V B 7 

Cornucopia VI F 5 

Cosie D. V 13 

Countess VI C 21 

Country Boy V D 60 

Coyne Placer HI A 10 

Cnpple Creek IV D 9 

Crcesus IV B 65 

Croesus No. VI D 22 

Crcesus No. 1 VI D 23 

Croesus No. 2 VI D 24 

Croesus No. 3 VI D 25 

Croesus No. 4 VI D 26 



Croesus No. 5 VI D 27 

Croesus No. 6 VI D 28 

Cross VD14 

Crown Placer VII B 4 

Crown Point HI D 36 

Crown Point VI E 8 

Cub VD23 

Cuba V0 38 

Cucumber Patch Placer Tract A 

VA33 
Cucumber Patch Placer Tract B 

VA31 
Cucumber Patch Placer Tract 

VAIO 
Cucumber Patch Placer Tract D 

VA30 
Cumberland V: D 41 
Czar VD15 

D VI 35 
Daisy V F 15 
Damascus HI 24 
Dandy V 47 
Dania HI D 1 
Dash Warn HI 11 
Davenport Placer II D 1 
Davis VD37 
Deadwood IV 26 
Deadwood V D 42 
Deer HI E 37 
Delaware HI D 42 
Delaware No. 1 HI D 43 
Delaware No. 2 HI D 44 
Delaware Placer II 2 
Delta VID62 
Delta Placer VG5 
Dennison Placer V A 38 
Denver IV D 8 
Denver VI E 33 
Denver City VII E 1 
Derry Placer VII G 1 
Detroit IV C 56 
Detroit Placer IV 36 
Devil VD61 
Dew Drop VI E 3 
Dewey Vl 66 
Dewey Placer VII A 2 
Dexter VI D 54 
Diamond Dick V 57 
Diana IV B 11 
Dictator No. 1 V G 7 
Dictator No. 2 VI G 6 
Dictator No. 3 VI G 7 
Dictator No. 4 V G 8 
Die Vernon V D 56 
Dirigo VC35 
Dirigo IVB85 
Discovery HI 10 
Discovery Ex. HI C 3 
Dix Placer VI A 6 
Dixie HI 7 
Doe HI E 35 
Don Juan HI E 5 
Dora L. IV A 26 
Double Ex. IV 24 
Double Header IV A 20 
Double Standard VII E 35 
Doubtful VII E 25 
Dreadnaught VI D 31 
Dreadnaught No. 1 VI D 30 
Dry Gulch Placer IV C 38 
Dry Placer VI D 11 
Du Lac Placer I B 2 
Dunedin V D 38 

E VI 43 
Eagg VI E 27 
Eagle IV B 5 
Eagle HID 49 



114 



GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 



Eagle VIE 28 
Eagle No. 3 VI E 26 
Eckhart Patch Placer IV G 1 
Eclipee VI D 14 
EclipeeNo. 1 VI D 13 
Eda IV B 4 
Edna V D 70 
Edna V F 16 

E. G. G. HID 9 
Egypt III C 26 
Egypt IVF18 
Eldorado V A 23 
Eldomdo V F 39 
Electra III D 26 
Elenora IV B 49 
Elephant V F 29 
Elizabeth IV B 46 
Elk VIC 3 

Elk HIE 38 
Elkhorn VI B 13 
Ella IV B 3 
Ella VD28 
EllaG. VE40 
Ellsworth IVD42 
Elyria M. S. IV A 23 
Emadin HI D 73 
Emilie Placer V D 59 
Emma IV D 12 
Emma VII E 37 
Emma V F 17 
Emma L. HI D 8 
Emmet HI E 26 
Emmett VI C 48 
Emperor V F 28 
Empire VI D 34 
Engle Placer V B 2 
Enterprise V F 53 
Ergo HID 18 
Erie IV F 6 
Erie IV F 13 
Ethelena IV C 18 
Eunice Bell V C 10 
Eureka IV B 78 
Eureka HI F 25 
Eva VIC 52 
Evans Placer VII A 1 
Evening Star VI C 26 
EvemngStar VI G 2 
Excelsior VI D 51 
Excelsior Placer HI B 7 
Excelsior Placer IV G 3 
Extenuate V C 53 
Extenuate No. 2 V C 52 

F VIC 42 
Fair Chance V E 22 
Fairview Placer HI B 3 
Faith VD6 

F. and D. Placer VI A 3 
Fanny Barrett V A 13 
Fanny Placer VI B 1 
Fargo VIC 67 

Fawn HI E 34 
Fellah HI C 27 
First Discovery VI D 40 
Flagstaff Vlfe43 
Floradora Placer VI B 7 
Florence HI C 38 
Florence IV D 39 
Florence IV F 10 
Florence VII F 4 
F. P. D. HI C 5 
Fog HIE 33 
Foote VI B 16 
Ford Placer V C 65 
Forest Queen V F 1 
Fourth of July VII C 2 
Fox Lake I V B 7 
Fraction IV B 19 
Fraction III C 15 



Fraction IV C 55 
Fraction HI D 37 
Fraction V D 35 
Fraction Placer V A 40 
Franklin IV B 81 
Frederick the Great V F 30 
Free and Easy HI D 74 
Free Gold HI E 6 
Free Gold No. 2 HI E 7 
Free Gold No. 3 HI E 8 
French IV D 17 
French Gulch Placer V B 12 
Friday V E 24 
Friendship IV A 9 
Fugitive VI C 83 

Fuller and Greenleaf Placer (Survey 
83) IVF19 

G VIC 41 

Gad HIE 51 

Galena HI D 17 

Galena II £ 5 

Galena Placer II D 3 

Garfield HI D 61 

Garfield HI E 42 

Gen. Butler HI D 81 

Gen. Grant VII D 14 

Gen. Jackson Placer VI D 43 

Georgetown Miner IV B 74 

George Washington IV B 27 

Georgia IV F 12 

Geoi^ie V C 45 

Germania IV A 29 

Germania VII B 23 

Gertie Corey HI E 69 

Giant HID 56 

Giantess HI D 57 

Girlie B. HI D 7 

Gissee HI E 29 

Gladstone V F 36 

Glenn VII E 39 

Glen wood HI C 13 

Gloucester IV D 30 

Gold Belle VI E 25 

Gold Belle No. 2 VII E 31 

Gold Brick HI D 54 

Gold Bug HID 22 

Gold Bug HID 51 

GoldC^in VI D 64 

Gold Cord HI D 55 

Gold Dust VIC 74 

Gold Dust Placer IV A 27 

Gold Eagle Placer HI A 1 

Gold Flake HI D 29 

Gold Hill IV D 19 

Gold Hill Placer IBl 

Gold King VII E 16 

Gold King No. 1 Placer VII B 2 

Gold King Placer VII B 1 

Gold Nugget VII E 17 

Gold Nugget Placer VII C 8 

Gold Nugget No. 2 Placer VII C 9 

Gold Pan VI D 44 

Gold Pan No. 2 VI D 59 

Gold Run IV D 16 

Gold Run No. 1 HI C 8 

Gold Run Placer IV C 1 

Gold Standard IV D 44 

Gold Wonder Placer I A 2 

Golden Age Placer HI H 4 

Golden Calf IV F 16 

Golden Crown Placer VII B 8 

Golden Edge VI E 42 

Golden Edge No. 2 VI E 45 

Golden Edge No. 3 VI E 40 

Golden Edge No. 4 VI E 41 

Golden Edge Placer VI E 32 

Golden Gate V E 28 

Golden Gate Placer IV D 56 

Golden Rule IV E 3 



Goldenrod VI F 4 
Governor King V E 15 
Gov. Waite I II D 38 
Grant HI D 59 
Graphic VIC 53 
Great Northern V D 21 
Great West V E 10 
Greenwood V D 41 
Grey Eagle III D 78 
Grey Horse IV D 58 
Ground Hog No. 1 V A 9 
Ground Hog No. 2 V A 8 
Ground Hog No. 3 V A 7 
Grouse VI C 63 
Grouse VI E 13 
Grubstake No. 1 HI D 67 
Grubstake No. 2 HI D 66 
Grubstake No. 3 III D 65 
Gulch HIE 48 
Guyot Placer VI G 5 

H VIC 40 

Halifax VI D 42 

Hamilton II E 2 

Hammer HI E 50 

Hanna V E 41 

Hannibal and St. Joe VI C 60 

Happy Jack V F 13 

Hard Luck IV C 59 

Harold Placer IV A 22 

Harrison IV B 70 

Harrison IV D 3 

Harry S. VII E 19 

Harum IV D 54 

Harum Placer IV E 2 

Hattie HI C 9 

Hattie M. IV B 42 

Havana V C 55 

Hayes IV B 10 

Hayes HI D 60 

Hazel Kirk V C 11 

Hazel Placer IV A 28 

H. B. D. HI C 42 

Helena VI D 47 

Helena No. 2 VI D 48 

Helena No. 3 VI D 49 

Helena No. 4 VI D 50 

Helena Placer VI D 46 

Helen B. VI C 79 

Helen G. Ill D 3 

Helen No. V D 65 

Helen No. 1 V D 67 

Helen Placer IV D 55 

Helens Baby V D 66 

Hematite IV D 21 

Hendrix IV C 27 

Hermit Placer VI B 6 

Hidden Treasure VI C 7 

Hidden Treasure No. 1 IV C 47 

Hidden Treasure No. 2 IV C 48 

High Five IV A 25 

Highland Mary VII E 15 

Highland Mary HI F 27 

HilLside VI C 15 

Hillside V F 7 

Hindoo IV B 37 

Hope VD5 

Hopeful V D 64 

Homegtake HI E 21 

Horton V E 42 

Humbui? IV F 11 

Hunt a Name V E 9 

Hunt Placer V A 36 

Huron Placer IV B 83 

I VIC 39 

Idaho VA25 

Ida M. HI F 15 

Ida M. No. 2 HI F 18 

Ida M. No. 3 HI F 17 



THE PBIMABY OBE DEPOSITS IN QENEBAL. 



116 



niinois VA14 

Illinois Placer VI C 73 

Illinois Placer VII D 3 

Ina VII E 38 

Independence VII C 1 

Independence VI D 8 

Independence Placer I B 4 

Indian Girl VI E 22 

Intrepid IV C 20 

Iowa V A 15 

Iowa IVD28 

Iowa Placer II D 2 

Ira Roberts IV B 8 

Irene C. Ill D 2 

Iron IV B 80 

Iron VII B 5 

Iron Mask IV A 24 

Iron Mask V C 34 

Iron Mask VG4 

Ironside III D 20 

Ironside IV D 37 

Israelite IV F 17 

I. X. C. O. Ill C 12 

I. X. L. Ill F 29 

I. X. L. Placer III F 37 

J VIC 38 

Jack IVB68 

Jackson M. S. V D 50 

James G. Carlisle V E 37 

Jane S. Ill D 4 

Janice IV D 47 

Jasper III E 19 

Jersey Placer V A 39 

Jerusalem Placer VI D 12 

Jessie III C 2 

Jessie III C 22 

Jessie IV C 10 

J. I.e. HID 64 

Jim Crow VI E 20 

Joe Davis III D 71 

JoeGliddon IV F 3 

Johannesburg V C 9 

Johannesburg Placer V C 69 

John Bell III D 14 

John J. Placer V E 4 

John Shock III C 25 

Johnson VII D 1 

Jove VII E 31 

Judson V F 10 

Julia VF40 

July HID 21 

Jumbo IV C 2 

June Bug IV (^ 43 

June Bug III D 27 

Juniata VI C 29 

Juniata Ex. VI C 24 

Jupiter HIE 57 

Juventa V E 18 

Juventa VII E 30 

K VIC 37 

Kansas HI D 77 

Kate IVB77 

Kate VII E 26 . 

Kate Els VII F 2 

Kate S. Placer II A 5 

Kathleen V D 33 

Kensington Placer Tract A IV C 23 

Kensington Placer Tract B V C 70 

Kentucky V D 53 

Kentucky VI D 15 

Kentucky Placer II C 1 

Ketman V C 29 

Keystone VI E 4 

Keystone VI E 19 

Key West V F 32 

Kimball Placer IBS 

King VI D 65 

Kingfisher VF48 



King Solomon VI E 12 
Kiowa IVB39 
Kit Carson V F 49 
KittieM. VI G 3 
Klack Gulch Placer VI B 5 
Klondyke V C 41 
Knob No. 1 VI E 51 
Knob No. 2 VI E 50 
Knob No. 3 VI E 49 
Knob No. 4 VI E 52 
Knob No. 5 VI E 53 
Knob No. 6 VI E 54 
Knorr III E 27 
K. T. HI D 13 

L VIC 36 

lAdy Huntington VI E 11 

Lady of the Mountain VII F 3 

Lafayette Placer III A 9 

Lafe Pence HI D 15 

Lake Placer VI A 4 

Lake Superior Placer IV B 30 

Lakota Placer V E 31 

Lakota Placer VI E 47 

Langdon V C 39 

Langdon No. 2 IV C 49 

Last Chance IV C 5 

Last Chance VI C 57 

Last Chance IIIF3 

Last Chance Ex. HI F 26 

Last Chance Placer VI D 21 

Last Dollar VF41 

Laura H HI D 10 

Laurium VI D 52 

Laurium No. 2 VI D 56 

Laurium No. 3 VI D 57 

Laurium Placer VII D 4 

Leadville IV D 10 

Ledge II E 6 

Legal Tender HI A 6 

Lennox VI D 1 

Leona IV B 75 

Leona V D 71 

LibbieK. IV F 7 

Liberty V D 10 

Liberty Placer II A 1 

Lightbum IV D 32 

Lightning V C 24 

Lightning No. 1 V C 25 

Lincoln IV B 9 

Lincoln HI D 58 

Lincoln VII D 15 

Lion VII E 27 

Little Betsey HI D 28 

Little Cally VI C 59 

Little Chief VI E 21 

Little Chief No. 2 VI E 48 

Little Corporal IV B 38 

Little Daisy V C 20 

Little Deber HI E 71 

Little Dick IV C 34 

Little Emy V F 51 

Little Harry IV D 18 

Little Harry IV D 36 

Little Joe IV C 33 

Little Lizzie VC49 

Little Maud IV B 55 

Little Morgan V F 25 

Little Sallie Barber VI D 38 

Little Sallie Barber Ex. V D 72 

Little Sarah VII E 8 

Little Tom VI C 64 

Little Tommie VI E 46 

Little Tommie No. 2 VII E 12 

Little Tommie No. 3 VII E 13 

Little Tommie No. 4 VII E 14 

Lizzie VI B 24 

Lizzie Moore V E 26 

Logan HI E 11 

Log Cabin VI G 1 



Lomax Gulch Placer V A 34 

Lone "Bub V D 69 

Lone Hand IV A 19 

Lone Star VI C 72 

Longfellow HI F 31 

Lookout HIE 18 

Loop IIIC29 

Lot No. 2 IV B 26 

Lot No. 3 IV B 24 

Lot No. 4 VII E 20 

Lot No. 4 IV B 22 

Lot No. 5 IV B 20 

Lottie B HI C 37 

Lottie May IV C 11 

Louisa V B 4 

Louis D. Placer V C 61 

Lucky VII B 7 

Lucky IV D 43 

Lucky VD51 

Lucile IVD34 

Lulu HIE 62 

Luna HIE 54 

Lyra HI E 61 

M VC68 

McKinley IV D 6 

McKinley VI D 63 

McKinley HI E 40 

McKinney V D 7 

McMc HIE 47 

Magic VF37 

Maggie VI C 23 

Maggie VII E 9 

Maggie Placer VI B 3 

Magnet IV C 52 

Magnet IV F 14 

Magnum Bonum Placer IV B I 

Maid of the District V D 63 

Mammoth IV D 35 

Mammoth HI F 10 

Manilla IV E 4 

Maple Leaf HI E 9 

Mary G. HI D 6 

Mary Gardner IV B 72 

Mars HIE 55 

Mascot IVC29 

Mascot Placer HI G 1 

Masonic Placer IV A 6 

Mastodon IV B 35 

Matchless V F 44 

Mathilda IV B 48 

Mattie V D 54 

Maurine IV D 48 

Mavoumeen V D 34 

Max VF54 

Maxwell VI C 13 

May VIB22 

May B. Ill C 16 

Maybell V C 46 

May W. IV B 44 

Melbourne IV D 61 

Me Next II E 7 

Merrimack HI D 76 

Merry Gold V D 16 

Metcalf Placer V A 37 

Me Too VI G 8 

Mexican VI D 20 

Michigan VI D 33 

Midnight Placer II E 8 

Miller VC42 

Miller Placer IV E 8 

Miller Placer IV G 2 

Milwaukee V C 16 

Mineola IV C 54 

Mineral Chief HI F 19 

Mineral Hill VE2 

Minnie V D 40 

Minnie B. VH G 3 

Minnie L. HI C 34 

Modoc VD62 



116 



GEOLOGY AND ORE DEPOSITS OF BBECKENBIDGE DISTRICT, COLORADO. 



Mogul VC27 
MollieB. 1VB18 
MoIlieB. HID 5 
Monarch VI CI 
Monitor IV C 51 
Monitor III D 80 
Monitor V E 21 
Monitor No. 1 V E 20 
Mono VC54 
Monroe Placer IBS 
Monument VII E 34 
Moonstone VI C 10 
Moose VIC 4 
Mooee III E 67 
Morningside IV A 14 
Morning Star IV B 36 
Morning Star VI C 27 
Morning Star VII E 33 
Morning Star VII E 36 
Morning Star III F 14 
Morning Star VI G 4 
Morse Placer I A 1 
Morton IV B 71 
Morton IV D 5 
Mountain Lion IV B 76 
Mountain Lion V G 6 
Mountain Pride VII D 13 
Mt. Nebo VIE 18 
Mt. Royal V F 38 
Muddy HID 53 
Mulberry Placer VG9 
Muldoon HID 79 
Muldoon III F 18 
Mumford No. 1 Placer II B 3 
Mumford No. 2 Placer II B 6 
Mumford No. 3 Placer III B 6 
Mumford No. 4 Placer III B 5 
Myrtle Annie V B 17 

N VC67 
Nahant IV C 17 
Nannie Houston VI B 21 
Naperville IV B 82 
Napoleon Placer III A 2 
Nebraska Placer VI F 2 
Neglected Placer VII D 5 
Nellie III E,75 
Nellie H. VID32 
Nellie Placer VI B 30 
Nellie Placer VII B 3 
Nelson III D 32 
Nelson Ex. Ill D 40 
Nevada V A 21 
New Discovery III F 39 
New England Placer VI B 14 
New Market VI D 61 
Newport III D 47 
New Year IV A 8 
New York IV D 25 
New York V E 34 
New York VII E 2 
New York No. 1 V B 18 
New York No. 2 V (' 1 
New York No. 2 VC8 
New York No. 3 VC2 
New York No. 4 V C 3 
New York No. 5 V C 6 
New York No. 6 VC7 
New York No. 8 VC4 
New York No. 9 V C 5 
Nickel Plate IV C 46 
Nil Desperandum III D 50 
North Star III E 56 
Nutmeg V D 27 

VC66 

Ocean Wave VI B 9 
Ocean Wave V E 13 
Ohio VA26 
Ohio Placer II C 3 



0. I. C. VII E 7 
0. J. Lewis III F 34 
0. K. IV D 38 
O.K. HIE 22 
O. K. V E 27 
Old Ironsides VI C 19 
Old Joe VF19 
Old Tennessee V D 45 
Old Tom IVC40 
Old Union V C 33 
Ontario V F 24 
Oregon IV D 27 
Oreomogo VI C 28 
Oro VC60 
Orthodox V D 17 
Orthodox No. 2 V D 19 
Orthodox No. 3 V D 20 
Orthodox M. S. V D 18 
Ouray VI C 55 
Outlet VD77 
Outlet Placer HI D 69 
Oxford VC40 

Pacific VIC 75 
Paducah V D 52 
Page VIC 17 
Parallel IV B 57 
Paris IVB54 
Park Placer VII C 7 
Patti VF2 
Paymaster HI F 2 
Peabody Placer HI CI 
Pearl V F 22 
Peerless V A 12 
Peerless V A 29 
Peerless HI E 76 
Pelican IV C 68 
Pennsylvania V A 32 
Peoria V D 47 
Pi VID67 
Pi No. 2 VID68 
Pi No. 3 VID69 
Pick HIE 49 
Pioneer HI E 25 
Pittabui^g Placer VI C 78 
Plow Boy IVB58 
Polar Bear VE5 
Pollux HIE 59 
Pontoon V F 52 
Pony Express VII E 18 
Populist HI D 39 
Prairie Dale HI E 4 
Price VIC 16 
Primrose Placer HI F 41 
Princeton IV B 50 
Prize Box VC66 
Producer VF8 
Protector Placer IV A 4 
Puzzle VI C 56 
Puzzler VG3 

Q VI D 6 

Quality Hill IV B 31 
Quartz Mountain IV A 15 
Quartz Mountain Placer II A 2 
Quartz Placer IV A 5 
Queen of the Forest V F 26 
Queen of the West IV C 31 
Queen Placer II D 4 

R VID5 

R. A. Gardner IV B 69 

Rain HI E 30 

Rankin Placer V B 1 

Red Mountain Placer II A 3 

Red Rover HI B 1 

Redwing VI C 12 

Regent V A 17 

Reindeer HI E 66 

Reliable V E 23 



Reservoir Placer HI D 41 
Reveille V F 35 
Revenue HI A 5 
Reynolds VI E 14 
Richmond V E 17 
Riddle Placer VI D 45 
Riley Placer II B 2 
Rising Moon V C 37 
Rising Sun VI E 5 
Riverside Placer VI B 2 
Robley V D 44 
Rochester V E 32 
Rochester VF43 
Rocky Point VII C 3 
Rocky Point Placer VII C 6 
Roger Q.Mills V E 38 
Rollins HIE 41 
Romance IV B 33 
Roosevelt No. 1 V C 21 
Roosevelt No. 2 V C 22 
Roosevelt No. 3 V C 23 
Rosa IVB29 
Rose F. IV B 43 
Rose Lee VI E 15 
Rose of Breckenridge V D 25 
Rose of Breckenridge V D 32 
Roeewater No. 1 V C 19 
Rosewater No. 2 V C 18 
Rosewater No. 3 V C 14 
Rosslyn IV B 63 
Royal Tiger HI F 28 
Rub VI C 76 
Ruth IV C 45 

S VI D 4 
Sadie HI E 64 
Salina IV B 32 
Sallie Barber V D 73 
Sallie LewiB HI F 24 
Sam Blair Placer HI A 3 
Sam Clark V D 36 
San Francisco IV C 12 
San Francisco No. 2 IV C 13 
San Francisco No. 3 IV C 14 
Sawmill Patch Placer V A 35 
Schley IV D 31 
Scott VIC 61 

Semper Idem Placer VII F 5 
Shakespeare IV B 2 
Shamrock IV B 84 
Shanm)ck VI E 35 
Shamrock No. 2 VI E 34 
Shekel Placer II B 5 
Sheridan IIIF4 
Sherman HI D 75 
Sherman HI E 39 
Siam V C 59 
Side Line IV A 18 
Sidney V C 26 
Sidney IV D 60 
Silent Friend V C 32 
Silver Boom IV B 14 
Silver Cup VI E 24 
Silver Cup No. 2 VI E 30 
Silver Dick VI B 10 
Silver Eel HI F 36 
Silver Group No. 1 VI B 25 
Silver Group No. 2 VI B 26 
Silver Group No. 3 VI B 27 
Silver Group No. 4 VI IJ 28 
Silver Group No. 5 VI C 80 
Silver Ilead^ V C 51 
Silver King VI D 10 
Silver Star V D 58 
Silverthom Placer V B 8 
Simcoe V C 36 
Sin San HI D 11 
Siphon VII C 4 
Sirius HI E60 
Sisler Placer Tract A V C 64 



THE PBIMAKY ORE DEPOSITS IN GENERAL. 



117 



Sifller Placer Tract B V B 16 

Slide VI 65 

Slide III 28 

Small Hopes IV C 4 

Small Spot IV C 8 

Smith Placer IV E 7 

Smooth III D 72 

Smuggler V C 31 

Smuggler III F 12 

Snider Placer VB5 

Snow-bank IV B 25 

Snow-drift V D 31 

Snow-storm IV B 53 

South America III F 1 

South Elkhom VI B 19 

South Etigrade VI B 18 

South Side Placer VI B 4 

St. James VI D 9 

St. John No. 1 VII E 23 

St. John No. 2 VII E 22 

St. John No. 3 VII E 21 

St. Lawrence IV C 57 

St. Louis VII B 6 

St. Louis VII D 8 

St. Louis No. 1 . Ill F 5 

St. Louis No. 2 IIIF6 

St. Louis No. 3 III F 7 

St. Paul IV B 6 

St. Paul VC17 

St. Paul VE3 

Standard VI 49 

Standard No. 1 IV 15 

Standard No. 2 IV 16 

Stanton No. 1 III F 9 

Stanton No. 3 II1F8" 

Stark IVB52 

Star Placer VI D 58 

Star Spangled Banner III D 83 

Stars and Stripes III D 84 

Stevens and Baker No. 1 VI E 57 

Stevens and Baker No. 2 VI E 56 

Stevens and Baker No. 3 VI E 55 

Stevens and Baker No. 4 VI E 58 

Stevens and Baker No. 5 VI E 59 

Stevens and Baker No. 6 VI E 60 

Stillson Patch Placer V C 62 

Stonewall Jackson IV B 23 

Storm III E 32 

Storms Placer III E 1 

Streng V D 76 

Sue III F 40 

Sue VF18 

Sultana IV B 12 

Summit IV A 12 

Summit III D 12 

Summit III D 34 

Summit IV D 13 

Summit No. 1 IV A 13 

Summit No. 2 IV A 11 

Summit Placer II E 3 

Sundown IV 6 

Sundown Placer VI A 5 



Sundown Placer IV 35 
Sunnyside V F 45 
Sunset IV A 7 
Swallow III F 30 
Swan III D 19 
Swan King III F 33 
Swans Nest Placer I C 1 

T VID3 

Tecumseh VI 54 

Tecumseh V D 30 

Teddy III F 35 

Telephone Girl V F 5 

Teller IV 30 

Tennessee III E 63 

Ten Strike IV A 16 

Terrible VII D 6 

Texas IV D 29 

T. H. Fuller Placer VI 31 

Thelma IV D 51 

Thistle IV 22 

Thornton V F 14 

Three Brothers IV B 73 

Three Links VI E 10 

Tiger VI B 8 

Tiger VII E 28 

Tiptop IVB34 

Toledo III 39 

Tom VEll 

Tom Price V D 29 

TosieC. IV F 9 

Treasure IV D 46 

Treasury IV F 1 

Treble Ex. IV 60 

Treble Ex. S. W. IV C 28 

Triangle III D 46 

Triangle V F 27 

Troy VI B 29 

Truax V D 12 

Tunnel VI E 16 

Twin Sistere VII E 10 

Tyra Placer VI A 2 

U VI D 2 
Uncle Sam VI 44 
Unicom Placer IV D 57 
Union V A 28 
Union V C 30 
Upper Ten VI F 3 

V VD75 
Valley III 18 
Valmy III E 68 
Vandalia VI 62 
Venus HIE 52 
Vienna Placer V F 47 
Vigilant VI E 36 
Virginia V A 22 
Virginia V 28 
Virginia V F 34 
Virginia City V E 35 
Virtotus Placer V E 1 



Virtotus Placer Lot A IV E 9 
Virtue VD3 
Volunteer IV 21 
Vermont V A 18 
Vulcan VII E 4 
Vulcan No. 1 VII E 3 
Vulcan No. 2 VII E 5 
Vulcan No. 3 VII E 6 

Walker VI D 53 
Walker Placer IV F 4 
Waltham No. 1 VII D 11 
Waltham No. 2 VII D 10 
Washington VI 47 
Washington Placer III A 8 
Wataon Placer VI 77 
Waters V D 46 
W. B. Stephenson IV B 13 
Weaver III D 16 
Weaver III E 46 
Webster III E 44 
Wellington V D 8 
Wellington No. 2 V D 11 
Wellington No. 3 V D 13 
Wellington Ex. V D 9 
Wellington Placer IV E 1 
West Laurium VII D 2 
West Point VI C 81 
Whale IVD33 
Wlieaton III D 63 
White Bear VE8 
White Cloud III D 23 
White Pine V D 43 
White Top VI E 9 
Wicklow IVB61 
WUdcatNo. 1 VA6 
Wildcat No. 2 VA5 
WOdcatNo. 3 VA4 
Wildcat No. 4 VA3 
Wildcat No. 5 VA2 
Wilderness IV F 15 
Williams Placer II F 1 
Williamsport VII D 12 
WUlieV IVD14 
Windsor VI 11 
Windy Gap Placer V F 56 
Wire Patch Placer VF31 
Wisconsin VI E 37 
Wolftone HID 30 
Wolftone VI 018 
Wood No. 1 III F 21 
Wood No. 2 III F 20 • 
Woodland Placer III E 77 
Wormwood IV D 52 
Wyneta IV 53 

X. 10. U. 8. V 53 

X. 10. U. 8. No 2 V 52 

Yellow Jacket III D 35 
Young America VII E 32 



Identification list of claims corresponding to coordinate squares and serial numbers in Plate II. 
Square I A. 



1. Morse Placer 

2. Gold Wonder Placer 

Square I B. 

1. Gold Hill Placer 

2. Du Lac Placer 

3. Monroe Placer 

4. Independence Placer 
6. Kimballp Placer 

Square I C. 
1. Swans Nest Placer 



Square n A. 

1. Liberty Placer 

2. Quartz Mountain Placer 

3. Red Mountain Placer 

4. Blue River Placer 

5. Kate S. Placer 

6. Band L. No. 1 Placer 

Square n B. 

1. Braddock Placer 

2. Riley Placer 

3. Mumford No. 1 Placer 

4. Bryan Placer 

5. Shekel Placer 

6. Mumford No. 2 Placer 



Square n C. 

1. Kentucky Placer 

2. Delaware Placer 

3. Ohio Placer 

Square n D. 

1. Davenport Placer 

2. Iowa Placer 

3. Galena Placer 

4. Queen Placer 

Square ZI S. 

1. Bed Rock Placer 

2. Hamilton 



118 



GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 



Square n E— Continued. 

3. Summit Placer 

4. Ballarat Placer 

5. Cralena 

6. Ledge 

7. Me Next 

8. Midnight Placer 

Square II F. 

1. Williams Placer 

2. American 

3. Central American 

Square m A. 

1. Gold Eagle Placer 

2. Napoleon Placer 

3. Sam Blair Placer 

4. Climax Placer 

5. Revenue 

6. L^gal Tender 

7. Annie Placer 

8. Washington Placer 

9. Lafayette Placer 
10. Coyne Placer 

Square m B. 

1. Red Rover 

2. Bonnie Nelson 

3. Fairview Placer 

4. Golden Age Placer 

6. Mumford No. 4 Placer 

6. Mumford No 3 Placer 

7. Excelsior Placer 

Square m C. 

1. Peabody Placer 

2. Jessie 

3. Discovery Ex. 

4. Arling 

5. F. P. D. 

6. C. B. and Q. 

7. Dixie 

8. Gold Run No. 1 

9. Hattie 

10. Discovery 

11. Dash Warn 

12. I. X. C. O. 

13. Glen wood 

14. Berlin 

15. Fraction 

16. May B. 

17. Baden Baden 

18. Valley 

19. Bullion 

20. Arab 

21. Bagdad 

22. Jessie 

23. Caliph 

24. Damascus 

25. John Shock 

26. Egypt 

27. Fellah 

28. Slide 

29. Loop 

30. Columbia 

31. Cecil C. 

32. Cora E. 

33. Bertha D. 

34. Minnie L. 

35. Clara L. 

36. Annie C. 

37. Lottie B. 

38. Florence 

39. Toledo 

40. Belle 

41. Chicago 

42. H. B. D. 



Square UZ D. 

1. Dania 

2. Irene C. 

3. Helen G. 

4. Jane S. 

5. MoUie B. 

6. Mary G. 

7. Girlie B. 

8. Emma L. 

9. E. G. G. 

10. Laura H. 

11. Sin San 

12. Summit 

13. K. T. 

14. John Bell 

15. Lafe Pence 

16. Weaver 

17. Galena 

18. Eigo 

19. Swan 

20. Ironside 

21. July 

22. Gold Bug 

23. White Cloud 

24. Camp Bird 

25. Badger 

26. Electra 

27. June Bug 

28. Little Betsey 

29. Gold Flake 

30. Wolftone 

31. Coin 

32. Nelson 

33. Bi-metallic 

34. Summit 

35. Yellow Jacket 

36. Crown Point 

37. Fraction 

38. Gov. Waite. 

39. Populist 

40. Nelson Ex. 

41. Reservoir Placer 

42. Delaware 

43. Delaware No. 1 

44. Delaware No. 2 

45. Bryan 

46. Tnangle 

47. Newport 

48. Albany 

49. Eagle 

50. Nil Desperandimi 

51. Gold Bug 

52. Blue Bird 

53. Muddy 

54. Gold Brick 

55. Gold Cord 

56. Giant 

57. Giantess 

58. Lincoln 

59. Grant 

60. Hayes 

61. Garfield 

62. Burlington 

63. Wlieaton 

64. J. I. C. 

65. Grubstake No. 3 

66. Grubstake No. 2 

67. Grubstake No. 1 

68. Birdseye No. 3 

69. Outlet Placer 

70. Clipper 

71. Joe Davis 

72. Smooth 

73. Emadin 

74. Free and Easy 

75. Sherman 

76. Merrimack 

77. Kansas 

78. Grey Eagle 

79. Muldoon 



Square in I>— Continued. 

80. Monitor 

81. Gen. Butler 

82. American 

83. Star Spangled Banner 

84. Stars and Stripes 

Square nz S. 

1. -Storms Placer 

2. Almira 

3. Clara 

4. Prairie Dale 

5. Don Juan 

6. Free Gold 

7. Free Gold No. 2 

8. Free Gold No. 3 

9. Maple Leaf 

0. Cache 

1. Logan 

2. ClSton 

3. Clifton M. S. 

4. Birdseye No. 2 

5. Birdseye No. 1 

6. Birdseye 

7. Berten 

8. Lookout 

9. Jasper 

20. Chester 

21. Homestake 

22. O. K. 

23. Boston 

24. Blue Bird 

25. Pioneer • 

26. Emmet 

27. Knorr 

28. Charlie W. 

29. Gissee 

30. Rain 

31. Cloud 

32. Storm 

33. Fog 

34. Fawn 

35. Doe 

36. Antelope 

37. Deer 

38. Elk 

39. Sherman 

40. McKinley 

41. Rollins 

42. Garfield 

43. Blaine 

44. Webster 

45. Clay 

46. Weaver 

47. McMc 

48. Gulch 

49. Pick 

50. Hammer 

51. Gad 

52. Venus 

53. Comet 

54. Luna 

55. Mars 

56. North Star 

57. Jupiter 

58. Castor 

59. Pollux 

60. Sirius 

61. Lyra 

62. Lulu 

63. Tennessee 

64. Sadie 

65. Caribou 

66. Reindeer 

67. Moose 

68. Valmy 

69. Gertie Corey 

70. Belle of Swan 

71. Little Deber 



THE PRIMABY ORE DEPOSITS IN OENEBAIi. 



119 



Square m E— Continued. 

72. Climax No. 4 

73. Climax No. 2 

74. Climax No. 3 

75. Nellie 

76. PeerlesB 

77. Woodland Placer 

Square m F. 

1. South America 

2. Paymaster 

3. Last Chance 

4. Sheridan 

5. St. Louis No. 1 

6. St. Louis No. 2 

7. St. Louis No. 3 

8. Staunton No. 3 

9. Staunton No. 1 

10. Mammoth 

11. Cashier M. S. 

12. Smuggler 

13. Cashier 

14. Morning Star 

15. IdaM. 

16. IdaM. No. 2 

17. Ida M. No. 3 

18. Muldoon 

19. Mineral Chief 

20. Wood No. 2 

21. Wood No. 1 

22. Brown Placer 

23. Amulet 

24. Sallie Lewis 

25. Eureka 

26. Last Chance Ex. 

27. Highland Mary 

28. Royal Tiger 

29. I. X. L. 

30. Swallow 

31. Longfellow 

32. British Boy 

33. Swan Kin^ 

34. 0. J. Lewis 

35. Teddy 

36. Silver Eel 

37. I. X. L. Placer 

38. Alice and Nellie Placer 

39. New Discovery 

40. Sue 

41. Primrose Placer 

42. Brewery Placer 

Square m O. 

I. Mascot Placer 

Square IV A. 

1. American Placer 

2. Adams Placer 

3. B. and L. No. 2 Placer 

4. Protector Placer 

5. Quartz Placer 

6. Masonic Placer 

7. Sunset 

8. New Year. 

9. Friendship 
10. Bull Dog 

II. Summit No. 2 

12. Summit 

13. Summit No. 1 

14. Momingside 

15. Quartz Mountain 

16. Ten Strike 

17. Beula 

18. Side Line 

19. Lone Hand 

20. Double Header 

21. Chesapeake 

22. Harold Placer 



Square IV A— Continued. 

23. Elyria M. S. 

24. Iron Mask 

25. High Five 

26. Dora L. 

27. Gold Dust Placer 

28. Hazel Placer 

29. Germania 

Square IV B. 

1. Magnum Bonum Placer 

2. Shakespeare 

3. Ella 

4. Edna 

5. Eagle 

6. St. Paul 

7. Fox Lake 

8. Ira Roberts 

9. Lincoln 

10. Hayes 

11. Diana 

12. Sultana 

13. W. B. Stephenson 

14. Silver Boom 

15. Breckenridge 

16. Como 

17. Cleveland 

18. Mollie B. 

19. Fraction 

20. Ix)t No. 5 

21. Bed Rock 

22. Lot No. 4 

23. StonewallJackson 

24. Lot No. 3 

25. Snow-bank 

26. Lot No. 2 

27. Geoi^e Washington 

28. Cleveland Placer 

29. Rosa 

30. Lake Superior Placer 

31. Quality HiU 

32. Salina 

33. Romance 

34. Tiptop 

35. Mastodon 

36. Morning Star 

37. Hindoo 

38. Little Corporal 

39. Kiowa 

40. Bumsides 

41. Bacon 

42. HattieM. 

43. RoseF. 

44. May W. 

45. Barbara 

46. Elizabeth 

47. Anna 

48. Mathilda 

49. Elenora 

50. Princeton 

51. Alliance 

52. Stark 

53. Snow-storm 

54. Paris 

55. Little Maud 

56. Bullion King 

57. Parallel 

58. Plow Boy 

59. Company 

60. Chantilly 

61. Wicklow 

62. Alice A. 

63. Rosslyn 

64. Canfield 

65. Croesus 

66. Athos 

67. Alice 

68. Jack 

69. R. A. Gaidner 



Square IV B— Continued. 

70. Harrison 

71. Morton 

72. Mary Gardner 

73. Three Brothers 

74. Georgetown Miner 

75. Leona 

76. Mountain Lion 

77. Kate 

78. Eureka 

79. Blue River 

80. Iron 

81. Franklin 

82. Naperville 

83. Huron Placer 

84. Shamrock 

85. Dirigo 

86. Company No. 2 

Square IV 0. 

1. Gold Run Placer 

2. Jumbo 

3. Buffalo 

4. Small Hopes . 

5. Last Chance 

6. Sundown 

7. Cleveland 

8. Small Spot 

9. Bessie 

10. Jessie 

11. Lottie Msiy 

12. San Francisco 

13. San Francisco No. 2 

14. San Francisco No. 3 

15. Standaid No. 1 

16. Standard No. 2 

17. Nahant 

18. Ethelena 

19. B. B. 

20. Intrepid 

21. Volunteer 

22. Thistle 

23. Kensington Placer, Tract A 

24. Double Ex. 

25. Consolidation 

26. Dead wood 

27. Hendrix 

28. Treble Ex. S. W. 

29. Mascot 

30. Teller 

31. Queen of the West 

32. Chance 

33. Little Joe 

34. Little Dick 

35. Sundown Placer 

36. Detroit Placer 

37. Carpenter Placer 

38. Dry Gulch Placer 

39. Buffalo Placer 

40. Old Tom 

41. Billie Button 

42. Blanche E. 

43. June Bug 

44. Blue 

45. Ruth 

46. Nickel Plate 

47. Hidden Treasure No. 1 

48. Hidden Treasure No. 2 

49. Langdon No. 2 

50. Chester 

51. Monitor 

52. Magnet 

53. Wyneta 

54. Mmeola 

55. Fraction 

56. Detroit 

57. St. Lawrence 

58. Pelican 



120 



GEOLOGY AND OBE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 



Square ZV D. 

1. Blaine 

2. Arthur 

3. Harrison 

4. Cleveland 

5. Morton 

6. McKinley 

7. Bryan 

8. Denver 

9. Cripple Creek 

10. Leaaville 

11. Breckenridge 

12. Emma 

13. Summit 

14. Willie V. 

15. A. Jay 

16. Gold Run 

17. Trench 

18. Little Harry 

19. Gold HUl 

20. Carbonate 

21. Hematite 

22. Cleopatra No. 1 

23. Cleopatra 

24. CleQpatra No. 

25. New York 

26. Brooklyn 

27. Oregon 

28. Iowa 

29. Texas 

30. Gloucester 

31. Schley 

32. Lightbum 

33. Whale 

34. Lucile 

35. Mammoth 

36. Little Harry 

37. Ironside 

38. O. K. 

39. Florence 

40. Buffalo 

41. Aepe 

42. EUsworth 

43. Lucky 

44. Gold Standard 

45. Climax 

46. Treasure 

47. Janice 

48. Maurine 

49. Ardath 

50. Boy 

51. Thelma 

52. Wormwood 

53. Carvel 

54. Harum 

55. Helen Placer 

56. Golden Gate Placer 

57. Unicorn Placer 

58. Grey Horse 

59. Brisbane 

60. Sidney 

61. Melbourne 
62.' Adelaide 

Square IV S. 

1. Wellington Placer 

2. Harum Placer 

3. Golden Rule 

4. Manilla 

5. Baldwin M. S. 

6. Baldwin 

7. Smith Placer 

8. Miller Placer 

9. Virtotus Placer, Lot A 



Square. IV F. 

1. Treasury 

2. Aye Kav 

3. Joe Gliadon 

4. Walker Placer 

5. Blind Tom 

6. Erie 

7. Libbie K. 

8. Annie B. 

9. Tosie C. 

10. Florence 

11. Humbug 

12. Georgia 

13. Erie 

14. Magnet 

15. Wilderness 

16. Golden Calf 

17. Israelite 

18. Egypt 

19. Fuller and Greenleaf Placer (Sur- 
vey 83) 

Square IV O. 

1. Eckhart Patch Placer 

2. Miller Placer 

3. Excelsior Placer 

Square V A. 

1. Boom Placer 

2. WUdcat No. 5 

3. WUdcat No. 4 

4. Wildcat No. 3 

5. Wildcat No. 2 

6. Wildcat No. 1 

7. Ground Hog No. 3 

8. Ground Hog No. 2 

9. Ground Hog No. 1 

10. Cucumber Patch Placer, Tract C 

11. Brooks-Snider M. S. 

12. Peerless 

13. Fannie Barrett 

14. niinois 

15. Iowa 

16. Bonanza 

17. Regent 

18. Vermont 

19. Clifton 

20. California 

21. Nevada 

22. Vimnia 

23. Eldorado 

24. Colorado 

25. Idaho 

26. Ohio 

27. Apex 

28. Union 

29. Peerless 

30. Cucumber Patch Placer, Tract D 

31. Cucumber Patch Placer, Tract B 

32. Pennsylvania 

33. Cucumber Patch Placer, Tract A 

34. Lomax Gulch Placer 

35. Sawmill Patch Placer 

36. Hunt Placer 

37. Metcalf Placer 

38. Dennison Placer 

39. Jersey Placer 

40. Traction Placer 

Square V B. 

1. Rankin Placer 

2. Engle Placer 

3. Columbia 

4. Louisa 



Square V B— Continued. 

5. Snider Placer 

6. Bartlett-Shock Placer 

7. Corkscrew Placer 

8. Silverthorn Placer 

9. Abbott Placer 

10. Bonanza Placer 

11. Blue River Placer 

12. French Gulch Placer 

13. Blue River 

14. Blue River No. 2 

15. Cleveland Placer 

16. Sisler Placer, Tract B 

17. Mjrrtle Annie 

18. New York No. 1 

19. Alice A. (M.S.) 

Square V 0. 

1. New York No. 2 

2. New York No. 3 

3. New York No. 4 

4. New York No. 8 

5. New York No. 9 

6. New York No. 5 

7. New York No. 6 

8. New York No. 2 

9. Johannesburg 

0. Eunice Bell 

1. Hazel Kirk 

2. Athol 

3. Cosie D. 

4. Rosewater No. 3 

5. Chicago 

6. Milwaukee 

7. St. Paul 

8. Rosewater No. 2 

9. Rosewater No. 1 

20. Little Daisy 

21. Roosevelt No. 1 

22. Roosevelt No. 2 

23. Roosevelt No. 3 

24. Lightning 

25. Lightning No. 1 

26. Sidney 

27. Mogul 

28. Virginia 

29. Ketman 

30. Union 

31. Smuggler 

32. Silent Friend 

33. Old Union 

34. Iron Mask 

35. Dirigo 

36. Simcoe 

37. Rising Moon 

38. Cuba 

39. Langdon 

40. Oxford 

41. Klondyke 

42. Miller 

43. Brown 

44. Charlie 

45. Georgie 

46. Maybell 

47. Dandy 

48. Boss 

49. Little Lizzie 

50. Colorado 

51. Silver Head 

52. X. 10. U. 8. No. 2 

53. X. 10. U. 8 

54. Mono 

55. Havana 
66. Prize Box 



THE PBIMABY OBE DEPOSITS IN GENEBAL.. 



121 



Square V C— Continued. 

57. Diamond Dick 

58. Chief 

59. Siam 

60. Oro 

61. Louis D. Placer 

62. Stillson Patch Placer 

63. Ada Placer 

64. Sisler Placer, Tract A 

65. Ford Placer 

66. O 

67. N 

68. M 

69. Johannesbuig Placer 

70. Kensington Placer, Tract B 

Square V D. 

1. Be^r Extension 

2. Bear 

3. Virtue 

4. Charity 

5. Hope 

6. Faith 

7. McKinney 

8. Wellington 

9. Wellington Ex. 

10. Liberty 

11. Wellington No. 2 

12. Tniax 

13. Wellington No. 3 

14. Cross 

15. Czar 

16. Merry Gold 

17. Orthodox 

18. Orthodox M. S. 

19. Orthodox No. 2 

20. Orthodox No. 3 

21. Great Northern 

22. Carrie La Salle 

23. Cub 

24. Brown M. S. 

25. Rose of Breckenridge 

26. Brownie Birdie 

27. Nutm^ 

28. Ella 

29. Tom Price 

30. Tecumseh 

31. Snow-drift 

32. Rose of Breckenridge 

33. Kathleen 

34. Mavoumeen 

35. Fraction 

36. Sam Clark 

37. Davis 

38. Dunedin 

39. Auckland 

40. Minnie 

41. Greenwood 

42. Dead wood 

43. White Pine 

44. Rol>ley 

45. Old Tennessee 

46. Waters 

47. Peoria 

48. Cassiopeia 

49. Cincinnati 

50. Jackson M. S. 

51. Lucky 

52. P^aducah 

53. Kentucky 

54. Mattie 

55. Buckeye 

56. Die Vernon 

57. Andromeda 

58. Silver Star 

59. Em i lie Placer 

60. Country Boy 



Square V D— Continued. 

61. Devil 

62. Modoc 

63. Maid of the District 

64. Hopeful 

65. Helen No. 

66. Helens Baby 

67. Helen No. 1 

68. Australia Gulch Placer 

69. Lone Bub 

70. Edna 

71. Leona 

72. Little Sallie Barber Ex. 

73. Sallie Barber 

74. Big Sallie Barber Ex. 

75. V 

76. Streng 

77. Outlet 

Square V B. 

1. Virtotus Placer 

2. Mineral Hill 

3. St. Paul 

4. John J. Placer 

5. Polar Bear 

6. Brown Bear 

7. Black Bear 

8. White Bear 

9. Hunt a Name 

10. Great West 

11. Tom 

12. Bill 

13. Ocean W'ave 

14. Ben Butler 

15. Governor King 

16. Celtic 

17. Richmond 

18. Juventa 

19. Christina 

20. Monitor No. 1 

21. Monitor 

22. Fair Chance 

23. Reliable 

24. Friday 

25. A No. 1 

26. Lizzie Moore 

27. O. K. 

28. Golden Gate 

29. Comet No. 2 

30. Cobb and Ebert Placer 

31. Lakota Placer 

32. Rochester 

33. Buffalo 

34. New York 

35. Virginia City 

36. Butte City 

37. James G. Carlisle 

38. Roper Q. Mills 

39. Clara W. 

40. Ella G. 

41. Hanna 

42. Horton 

Square V F. 

1. Forest Queen 

2. Patti 

3. Bess 

4. Comet No. 1 

5. Telephone Girl 

6. Comet 

7. Hillside 

8. Producer 

9. Abby 

10. Judson 

11. Comet No. 3 

12. Comet No. 4 

13. Happy Jack 



Square V F— Contlniiad. 

14. Thornton 

15. Daisy 

16. Edna 

17. Emma 

18. Sue 

19. Old Joe 

20. Conclave 

21. Arraria 

22. Pearl 

23. Comet 

24. Ontario 

25. Little MoTcan 

26. Queen of the Forest 

27. Triangle 

28. Emperor 

29. Elephant 

30. Frederick the Great 

31. Wire Patch Placer 

32. Key West 

33. Boss 

34. Virginia 

35. Reveille 

36. Gladstone 

37. Magic 

38. Mt. Royal 

39. Eldorado 

40. Julia 

41. Last Dollar 

42. Albany 

43. Rochester 

44. Matchless 

45. Sunnyside 

46. Clark Placer 

47. Vienna Placer 

48. Kingfisher 

49. Kit CarBon 

50. Belle 

51. Little Emy 

52. Pontoon 

53. Enterprise 

54. Max 

55. Climax 

56. Windy Gap Placer 

Square V O. 

1. American Placer (Survey 85) 

2. American Placer (Survey 84) 

3. Puzzler 

4. Iron Mask 

5. Delta Placer 

6. Mountain Lion 

7. Dictator No. 1 

8. Dictator No. 4 

9. Mulberry Placer 

Square VX A. 

1. Ada Placer 

2. Tyra Placer 

3. F. and D. Placer 

4. Lake Placer 

5. Sundown Placer 

6. Dix Placer 

Square VZ B. 

1. Fanny Placer 

2. Riverside Placer 

3. Maggie Placer 

4. Scjuth Side Placer 

5. Klack Gulch Placer 

6. Hermit Placer 

7. Floradora Placer 

8. Tiger 

9. Ocean Wave 
10. Silver Dick 



122 



GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 



Square VI B— Continued. 

11. Autocrat 

12. Autocrat Ex. 

13. Elkhorn 

14. New England Placer 

15. Combination 

16. Foote 

17. Conrad 

18. South Etigrade 

19. South Elkhorn 

20. Contact 

21. Nannie Houston 

22. May 

23. German la 

24. Lizzie 

25. Silver Group No. 1 

26. Silver Group No. 2 

27. Silver Group No. 3 

28. Silver Group No. 4 

29. .Troy 

30. Nellie Placer 

Square VZ C. 

1. Monarch 

2. Badger 

3. Elk 

4. Moose 

5. Ballard 

6. Chicago 

7. Hidden Treasure 

8. Bright Hope 

9. Chippewa 
1^0. Moonstone 

11. Windsor 

12. Redwing 

13. Maxwell 

14. Commodore Placer 

15. Hillside 

16. Price 

17. Page 

18. Wdftone 

19. Old Ironsides 

20. Ben Harrison 

21. Countess 

22. Aggie 

23. Ma^ie 

24. Juniata Ex. 

25. Bertie H. 

26. Evening Star 

27. Morning Star 

28. Oreomogo 

29. Juniata 

30. Bed Rock Placer 

31. T. H. Fuller Placer 

32. A 

33. B 

34. C 

35. D 

36. L 

37. K 

38. J 

39. I 

40. H 

41. G 

42. F 

43. E 

44. Uncle Sam 

45. Berlin Placer 

46. Atlantic 

47. Washington 

48. Emmett 

49. Standard 

50. Bulwer 

51. Anna 

52. Eva 

53. Graphic 

54. Tecumseh 

55. Ouray 



Square VI C— €k>ntlnued. 

56. Puzzle 

57. Last Chance 

58. Cliff 

59. Little Callie 

60. Hannibal and St. Joe 

61. Scott 

62. Vandalia 

63. Grouse 

64. Little Tom 

65. Slide 

66. Dewey 

67. Fargo 

68. Conquest 

69. Confidence 

70. Conjecture 

71. Compromise 

72. Lone Star 

73. Illinois Placer 

74. Gold Dust 

75. Pacific 

76. Rub 

77. Watson Placer 

78. Pittsburg Placer 

79. Helen B. 

80. Silver Group No. 5 

81. West Point 

82. Carpenter Placer 

83. Fugitive 

Square VZ D. 

1. Lennox 

2. U 

3. T 

4. 8 

5. R 

6. Q 

7! Bunker Hill 

8. Independence 

9. St. James 

10. Silver King 

11. Dry Placer 

12. Jerusalem Placer 

13. Eclipse No. 1 

14. Eclipse 

15. Kentucky 

16. Alameda 

17. California 

18. Belcher 

19. Cadiz 

20. Mexican 

21. Last Chance Placer 

22. Croesus No. 

23. Croesus No. 1 

24. Croesus No. 2 

25. Croesus No. 3 

26. Croesus No. 4 

27. Croesus No. 5 

28. Croesus No. 6 

29. Bellevue 

30. Dreadnaught No. 1 

31. Dreadnaught 

32. Nellie H. 

33. Michigan 

34. Empire 

35. Ann Arbor Placer 

36. Ann Arbor 

37. Brown 

38. Little Sallie Barber 

39. Big Sallie Barber 

40. First Discovery 

41. Cumberland 

42. .Halifax 

43. Gen. Jackson Placer 

44. Gold Pan 

45. Riddle Placer 

46. Helena Placer 

47. Helena 



Square VZ D— Continued. 

48. Helena No. 2 

49. Helena No. 3 

50. Helena No. 4 

51. Excelsior 

52. Laurium 

53. Walker 

54. Dexter 

55. Alice 

56. Laurium No. 2 

57. Laurium No. 3 

58. Star Placer 

59. Gold Pan No. 2 

60. Alpha Placer 

61. New Market 

62. Delta 

63. McKinley 

64. Gold Coin 

65. King 

66. Big Pan 

67. Pi 

68. Pi No. 2 ' 

69. Pi No. 3 

Square VZ E. 

1. Cecil 

2. Arthur Nail 

3. Dew Drop 

4. Keystone 

5. Rising Sun 
8. Alte 

7. Clara Belle 

8. Crown Point 

9. \MiiteTop 

10. Three Links 

11. Lady Huntington 

12. King Solomon 

13. Grouse 

14. Reynolds 

15. Rose Lee 

16. Tunnel 

17. Belle of Baloy 

18. Mount Nebo 

19. Keystone 

20. Jim Crow 

21. Little Chief 

22. Indian Girl 

23. Black Prince 

24. Silver Cup 

25. Gold Belle 

26. Eagle No. 3 

27. Eagg 

28. Eagle 

29. Black Prince No. 2 

30. Silver Cup No. 2 

31. Gold Belle No. 2 

32. Golden Edge Placer 

33. Denver 

34. Shamrock No. 2 

35. Shamrock 

36. Vigilant 

37. Wisconsin 

38. B. V. 

39. Blaine 

40. Golden Edge No. 3 

41. Golden Edge No. 4 

42. Golden Edge 

43. Flagstaff 

44. Blackhawk 

45. Golden Edge No. 2 

46. Little Tommie 

47. Lakota Placer 

48. Little Chief No. 2 

49. Knob No. 3 

50. Knob No. 2 

51. Knob No. 1 

52. Knob No. 4 

53. Knob No. 5 



THE PRIHABY OBE DEPOSITS IN GENEBALu 



123 



Square VZ E— Continued. 



54. Knob No. 

55. Stevens & 

56. Stevens & 

57. Stevens & 

58. Stevens & 

59. Stevens & 

60. Stevens & 

61. Carbonate 



6 

Baker No. 3 
Baker No. 2 
Baker No. 1 
Baker No. 4 
Baker No. 5 
Baker No. 6 
No. 2 



Square VZ F. 

1. Capt. Ryan Placer 

2. Neoraska Placer 

3. Upper Ten 

4. Goldenrod 

5. Cornucopia 

Sqoare VZ O. 

1. Log Cabin 

2. Evening Star 

3. KittieM. 

4. Morning Star 

5. Guyot Placer 

6. Dictator No. 2 

7. Dictator No. 3 

8. Me Too 

Sqoare VZZ A. 

1. Evans Placer 

2. Dewey Placer 

Square VZZ B. 

1. Gold King Placer 

2. Gold King No. 1 Placer 

3. Nellie Placer 

4. Crown Placer 

5. Iron 

6. St. Louis 

7. Lucky 

8. Golden Crown Placer 



Square VZZ C. 

1. Independence 

2. Fourth of July 

3. Rocky Point 

4. Siphon 

5. Birdie Treble 

6. Rocky Point Placer 

7. Park Placer 

8. Gold Nugget Placer 

9. Gold Nugget No. 2 Placer 

Square VZZ D. 

1. Johnson 

2. West Laurium 

3. Illinois Placer 

4. Laurium Placer 

5. Neglerted Placer 

6. Terrible 

7. Annex Placer. 

8. St. Louis 

9. Chicago 

10. Waltham No. 2 

11. Waltham No. 1 

12. Williamsport 

13. Mountain Pride 

14. Gen. Grant 

15. Lincoln 

Square VZZ S. 

1. Denver City 

2. New York 

3. Vulcan No. 1 

4. Vulcan 

5. Vulcan No. 2 

6. Vulcan No. 3 

7. O. I. C. 

8. Little Sarah 

9. Maggie 

10. Twin Sisters 

11. Baker 



Square VZZ E— Continued. 

12. Little Tommie No. 2 

13. Little Tommie No. 3 

14. Little Tommie No. 4 

15. Highland Mary 

16. Gold King 

17. Gold Nugget 

18. Pony Express 

19. Harry S. 

20. Lot 4 

21. St. John No. 3 

22. St. John No. 2 

23. St. John No. 1 

24. Carbonate 

25. Doubtful 

26. Kate 

27. Lion 

28. Tiger 

29. Ajax 

30. Juventa 

31. Jove 

32. Young America 

33. Morning Star 

34. Monument 

35. Double Standard 

36. Morning Star 

37. Emma 

38. Ina 

39. Glenn 

Square VZZ F. 

1. Convent 

2. KateElo 

3. Lady of the Mountain 

4. Florence 

5. Semper Idem Placer 

Square VZZ O. 

1. Derry Placer 

2. Bonanza 

3. Minnie B. 



CHAPTER IX. 
VEINS OF THE ZINC-LEAD-SILVER-GOLD SERIES. 

OENERAIi FEATURES. 

PliAK OF BESCBIFTIOX. 

Owing to the condition of most of the mines in the district, full description of the zinc- 
lead-silver-gold veins is very neariy confined to an accoimt of what may be seen in the Wellington 
workings. In order to avoid repetition it appears best under these circumstances, after a brief 
summary of some of the common characteristics belonging to the veins of this group, to proceed 
at once to a description of the Wellington ore bodies as constituting the most important examples 
of the type now open to investigation. This will be followed by such brief notes as may be 
presented concerning other mines whose ore bodies belong to the same group. 

BISTBIBTTTIOX. 

The general distribution of the zinc-lead-silver-gold veins and their close connection with 
the monzonite porphyry was indicated in Chapter VIIL All lie in the southwest half of the 
mapped area and nearly all are within or very near the great mass of monzonite porphyry that 
makes up most of Bald Mountain, together with Nigger, Prospect, and Mineral hUls. On 
Prospect and Mineral hills are the veins of the Wellington, Truax, Ella, Minnie, Cincinnati, 
Lucky, ai^d Melbourne mines, all traversing monzonite porphyry and all typical members of 
the class. Across French Gulch are the Dunkin, Juniata, and associated veins in the monzonite 
porphyry of Nigger Hill; the SalUe Barber vein, on Bald Mountain, in porphyry; and the 
Country Boy and Washington veins, traversing porphyry and quartzite. South of Illinois 
Gulch are the Puzzle-Ouray and Gold Dust veins, in monzonite porphyry, quartzite, and shale. 
Of the Mountain Pride vein little is known, but it apparently belongs to the zinc-lead-silver- 
gold series. It differs from the others mentioned in having the "Wyoming'* formation for its 
general country rock, with some monzonite porphyry, as the dumps show. On the north side 
of Gibson Hill are the Jumbo and accompanying veins, virtually at the northern limit of the 
monzonite porphyry. These veins consist of oxidized gold ore near the surface and change to 
pyritic deposits below. It is questionable whether they belong to the same class as the other 
veins mentioned, but too little can be learned about them to set them up as a distinct type. 
In some respects they appear to be intermediate between veins like those of the Puzzle mine 
and deposits like those of the Jessie mine. 

ATTITUDE. 

The general northeast strike of the veins and their various dips have already been men- 
tioned. To this group belong all the large deposits of distinctly veinlike form found in the 
district except the Laurium vein in the pre-Cambrian. In this coimection it is well to remember 
that classification is artificial, and that some of the lodes in the group now being described may 
extend down into the pre-Cambrian rocks and if the region were more deeply eroded would be 
put in the same class as the Laurium vein. 

BELATIOX TO FAULTING. 

It does not appear that any of the veins of the zinc-lead-silver-gold series occupy the fissures 
of important faults. Some movement undoubtedly took place when the fissures were first opened, 
for many of them were partly filled with brecciated and triturated material that was afterward 

124 




INCRUSTATION OF SIDERITE IN A VUG IN SPHALERITE. 

Natural size. See page 87. 



1 


^^I^P^^ffi^^H 


!^H 


1 






1 







A VEINLETS OF SIDERITE IN ORE COMPOSED CHIEFLY OF DARK SPHALERITE. 

Natural size. See page 87. 

ORE FROM WELLINGTON MINE. 



VEINS OF THE ZINC-LEAD-SILVER-GOLJ) SERIES. 125 

replaced by sulphides. Moreover, the movement continued during and after the period of ore 
deposition; but careful mapping has failed to show that the distribution of the rocks as seen 
at the surface was appreciably affected by this movement. Although the veins fill fault fissures, 
the faulting was negligible so far as the general structure of the district is concerned. This 
Ls not at all peculiar to this region but appears to be a relation between fissuring and ore depo- 
sition that is exceedingly common. Great fault fissures are rarely filled by veins, perhaps chiefly 
because great movement tends to pack the fissure tightly with compressed impervious attrition 
material, and if ore is deposited at all it is likely to be crushed and dispersed through the gouge 
by the recurrent movements characteristic of great dislocations. On the other hand, productive 
veins often accompany great faults and fill tlie minor associated fissures. 

WIDTH. 

The width of the veins ranges up to a maximum of about 15 feet, attained by the Siam 
vein. As a whole, the stopes on the Mineral Hill veins range from 4 to 10 feet. The unusually 
wide parts of the veins are as a rule not all ore but generally c on tain one or more horses of waste. 
The single stope opened in 1909 on the Country Boy vein showed a maximum width of 5 feet. 
One drift in the Gold Dust workings was carried in places to a width of 20 to 24 feet, but some 
of the miners who ran it report, apparently with truth, that the vein did not attain anything 
like that width. 

MATEBIAL. 

« 

The vein material of this group varies widely in composition. As a rule, the sulphides 
predominate and such gangue as may be present is ordinarily a carbonate approaching siderite 
in composition. Quartz is ahnost absent from most of the veins, but the ore of the Gold Dust 
vein is in part siliceous, although the quartz is cryptocrystalline and represents the silicification 
of crushed quartzite and shale rather than the free development of quartz in open fissures. A 
similar cryptocrystalline quartz gangue was noted also in a small spur from the Puzzle vein, in 
quartzite. One of the simplest vein fillings is that of the Country Boy, where stoping was in 
progress in 1909. Here the material is chiefly dark sphalerite in massive aggregation with 
wholly subordinate siderite and a little pyrite and galena. Toward the ends of the pay shoot 
this ore becomes more pyritic. In some of the upper workings also galena was more abundant 
than in the part of the vein now worked. The Wellington veins are generally filled with a 
massive aggregate of galena, sphalerite, and pyrite in various proportions, with a very small 
proportion of siderite. In some places the veins contain essentially a lead ore; in others a zinc 
ore. The galena is invariably argentiferous and its quantity is generally a good index of the 
proportion of silver in the ores. The ores from the French Creek mines belonging to the zinc- 
lead-silver-gold series do not carry much gold, the ore from the Wellington rarely containing as 
much as a tenth of an ounce to the ton. On the other hand, the Gold Dust and Puzzle ore 
may contain from 20 to 25 ounces of silver and up to an ounce of gold to the ton. Some 
pyritic concentrates from the Gold Dust vein are stated to have yielded 17 ounces of silver and 
2.5 ounces of gold to the ton. 

The veins exhibit scarcely any regularity in the arrangement of their constituents, which 
are as a rule confusedly and intimately mingled in one stnictureless aggregate. In parts of 
the Siam and other veins of the Wellington group the ore is banded, but this is due not to 
any original order of deposition of the sulphides but to a late fissuring of the ore parallel with 
its walls and an infiltration of siderite or other carbonate into the small cracks thus formed. 
A specimen of ore traversed by one of these later veinlets is shown in Plate XXV, B, Although 
the ore of the zinc-lead-silver-gold veins rarely shows conspicuous brecciation, its structure in 
many places suggests that the deposits of brittle sulphides with their many surfaces of contact 
have from time to time been fractured and loosened by sUght movements along the vein fissure 
and that fresh deposition of sulphides has then taken place, accompanied probably by some 
solution also of the older sulpliides. Thus the whole deposit, wliile not showing any regular 
or definite succession of younger generations of minerals upon older ones, represents the final 
result of a complex series of depositions following closely upon each period of fracturing. 



126 GEOLOGY AND ORE DEPOSITS OP BBECKENBIDGB DISTRICT, COLORADO. 

LATE MOVEMENTS. 

The veins generally are not much broken by recent movement and are not accompanied 
by much gouge. In many places the sulphides are adherent to both walls, even where the 
veins are several feet wide. In some places, however, there is a seam of clay gouge along one 
or both sides of the ore. This material varies with the country rock but is generally a tough 
unctuous clay some of which contains many minute crystals of pyrite that apparently have 
formed in the gouge itself. 

The veins are here and there displaced by later cross faults, as in the Wellington mine 
(p. 131), where there are two sets of such faults. Near these there is likely to be much local 
disturbance, the ore being broken and traversed by fissures filled with gouge. Unfortunately 
the conditions for studying displacements of the veins are in this district exceedingly unsatis- 
factory, the Wellington being the only mine that is at all illuminating as regards late faulting. 

OXIDATIOX. 

The general subjects of oxidation and enrichment are treated in another place. It will 
suffice to note here that the veins of the group under consideration oxidize as a rule to soft, 
earthy lead-carbonate ores, many of which are rich in silver. Exceptionally there is a notable 
concentration of gold near the surface. One of the first signs of oxidation in these veins is the 
occurrence of smithsonite in amorphous spongy form in the sphalerite, as shown in Plate 
XXVI, A. 

EXAMPLES OF ZINC -LEAD -SILVER- GOLD DEPOSITS. 

WELLXNOTON MINE. 
INTRODUCTION. 

The Wellington mine (see PI. XXVII, A), owned by the Wellington Mines Co., the stock 
of which is held largely in Kansas City, Mo., is situated on the north side of French Gulch, 2 
miles east of Breckenridge. The property represents a consolidation of the Oro and original 
Wellington mines, which was effected in 1902 by the organization of the Colorado & Wyoming 
Development Co. The present corporation, capitaUzed at $10,000,000, acquired the mines 
about five years later, the sale of stock being helped by very alluring reports of the mines' 
resources. The Oro appears to have become productive about the year 1887, and in 1890 it 
was yielding more ore than any other mine in the district. The Wellington was opened shortly 
after the Oro but never attained the same prominence as its neighbor. The early shipments 
were argentiferous lead ore, including both cerusite and galena, the zinc ore at that time being 
regarded as waste. The total production of the two mines prior to their acquisition by the 
present company is not accurately known but is generally supposed to have been about 
$1,500,000. 

In 1909 the Wellington Mines Co. was shipping daily about 25 tons of galena concentrates 
and from 8 to 10 tons of middlings consisting chiefly of sphalerite and pyrite. Shipments of 
crude ore and of high-grade sphalerite concentrates are also made from time to time. The first- 
class galena concentrates are bought by the Chamberlain-Dillingham Ore Co. and sliipped to the 
smelters of the American Smelting & Refining Co. Concentrates containing over 30 per cent 
of zinc and generally under 10 per cent of iron go to several smelters in Missouri, Oklahoma, 
and Kansas. The middlings are bought by the Western Chemical Manufacturing Co., of Denver, 
which pays for the zinc and uses the pyrite for making acid. 

The present mill, built in 1908, is situated near the portal of the Oro level. It is equipped 
with a jaw crusher, three sets of Morse rolls. Callow screens, six 6-compartment Harz jigs, and 
four Wilfley tables. In general the products are (1) galena concentrates; (2) the so-called 
'^middlings," including sphalerite concentrates that are purposely mixed with material contain- 
ing much pyrite; and (3) tailings, in which is included pyritic material from the third 
compartment of each jig. 



U. S. QEOLOaiCAL SURVEY 



PHOFESSIONAL PAPER 76 PLATE X 




A. SPONGY SMITHS0N1TE IN SPHALERITE, SALL1E BARBER MINE. 
Natural size. See page 88. 



Iff ,»• , ^j- «i . ^ I 







'-•■t£.' 



ORE STRUCTURES. 



VEINS OF THE ZINOLEAD-SILVER-GOLD SERIES. 127 

UNDERGROUND DEVELOPMENT. 

The plan of underground development of the Wellington mme is shown in Plate XXVIII. 
The general trend of the veins is N. 37** E. and they have been explored for a total length of 
2,600 feet. At the southwest end of the workings is the Oro shaft, with its collar about 50 feet 
above French Creek. This is connected with four levels at depths successively of 116.5, 185.6, 
245.8, and«323.8 feet. A large part of the product from the original Oro mine came from these 
levels, especially from stopes above the first level, which is about 1,100 feet in length. At 
present, however, the shaft is filled with water and practically all of the mining is confined* to 
two adit levels. The lower of these, known "as the Mill tunnel or Oro level, has its portal near 
the mill, about 160 feet west of and 5.5 feet lower than the collar of the Oro shaft. This level 
is a crosscut for 100 feet and thence follows the Siam vein for over 1,900 feet to the northeast, 
with one important branch to the east on the Orthodox vein. Originally the portal of this level 
was about 100 feet southwest of the Oro shaft, the level following the Oro vein (see PL XXVIII) 
for about 650 feet and then crosscutting north to the Siam vein. This old drift, although no 
longer used, is still open, but the crosscut to the north is filled. 

About 100 feet above the Oro level is the Extenuate * level, the portal of which is 1,600 
feet northeast of the portal of the Oro level. Recently an electric tramway has been built 
between the two adits. The Extenuate level consists of a straight crosscut running 800 feet 
in a N. 25** 13' W. direction, with right-hand and left-hand drifts on the principal veins. The 
upper of these two main levels, which are connected by a few winzes and raises, is only partly 
superposed over the lower, the lower one extending farther to the southwest and the upper one 
farther to the northeast. This fact, taken in connection with the absence or present inaccessi- 
biUty of workings above or below the main levels, confines the study of the mine so nearly to a 
horizontal plane as to afford Uttle opportunity for working out details of faulting or for observing 
possible changes in the ore bodies at different depths. About 227 feet above the Extenuate level 
is the old Wellington timnel, now abandoned and caved. East of it and about 50 feet lower is 
the Liberty tunnel, also inaccessible. The Siam tunnel, in the southwest part of the workings, 
and most of the other shorter timnels above the Oro level can no longer be entered. The Welling- 
ton Mines Co. apparently has no stope maps and its general map is not complete, especially 
in its representation of disused workings. A large part of the Extenuate level east of the main 
crosscut could not be examined in 1909 owing to the caving of the drifts or to the presence of 
foul air. 

GEOLOGIC RELATIONS. 

The general country rock of the Wellington mine is monzonite porphyry, being part of the 
same very irregular intrusive sheet that forms the upper part of Bald Mountain. The under 
surface of this property is not at present exposed in the WeUington workings, but the Oro shaft 
went into the underlying sediments and the general geologic relations brought out in the map- 
ping of this part of French Gulch, although indicative of -much irregularity, suggest that the 
bottom of the mass as a whole does not he much below the grade of French Creek. The visible 
body of porphyry, however, may be connected with dikes that extend indefinitely downward. 
Just west of the Oro shaft is the mass of quartzite (see PI. I, in pocket) in which are the Old 
Union workings. With this the porphyry is in ragged igneous contact. About 450 feet east 
of the Extenuate tunnel some of the same quartzite, including here, as near the Old Union 
mine, beds of compact, more or less jasperoidal limestone, is exposed in the lower Brown tunnel. 
At the Wellington mill is a tunnel that was open in 1908 but has since been closed by the con- 
struction of the mill and is not shown on the mine map (PL XXVIII). For about 100 feet 
this tunnel penetrates disturbed decomposed porphyry that is possibly talus. It then enters 
black shale and continues in this rock for 300 to 400 feet to the face. The bedding of the shale, 
although more or less disturbed, is on the whole nearly horizontal. No satisfactory informa- 
tion could be obtained in 1908 and 1909 regarding the distribution of rocks found in the Oro 

1 This name as recorded and as used on mine mapts Is written X. 10. U. 8, a form typographically so inconvenient as to Justify the substitution 
here employed in the text. 



128 GEOLOGY AND ORE DEPOSITS OP BBECKENRIDGE DISTRICT, COLORADO. 

shaft and in the levels connected with it. That some porphyry occurs on these levels is known, 
but they are largely in sedimentary beds. The part of the second level in the vicinity of the 
shaft, including most of the long northeast crosscut, appears from certain annotations on the 
company's maps to be in shale. Limestone is noted near the end of this crosscut and porphyry 
apparently constitutes the rock of the main drift eastward from a point about 25 feet east of the 
beginning of the northeast crosscut. The third and fourth levels are presumably in shale, or 
perhaps partly in quartzite, but no definite confirmation of this supposition could bfe obtained. 

That these various bodies of sedimentary rock may be isolated blocks included in the por- 
phyry is of course a possibility, this suggestion receiving some color of probability from the 
known occurrence of similar though generally smaller inclusions elsewhere in the district; but 
the numerous exposures of shale or quartzite, more or less covered by porphyry talus, along 
the bases of the slopes on both sides of French Gulch (see PL I) and the fact that the gravels 
along this stream are known to rest in many places on the sedimentary rocks indicate that the 
creek has cut through the main body of the porphyry and that further deepening of the channel 
would show only relatively small intrusive masses or dikes of the igneous rock. 

The prevailing porphyry of the Wellington mine, although apparently a continuous part of 
the large mass exposed on Mineral and Prospect hills, as well as in the tunnels of the Rose of 
Breckenridge, Minnie, and Lucky mines, is for the most part an unusually fine grained and incon- 
spicuously porphyritic facies. Chemical analyses of the typical fresh and altered porphyry of 
the mine accompanied by petrographic descriptions are contained in Chapter III (pp. 43-62). 
Much of the rock is almost aphanitic in texture, but some of the fresher varieties show abundant 
small phenocrysts of biotite and are characterized microscopically by the presence of both 
augite and hypersthene. Such fresh material, however, is exceptional and most of the rock 
seen in the mine is considerably altered by ore-depositing solutions complicated in many places 
by subsequent weathering. The nature of these processes of alteration has been described 
in Chapter VII (pp. 93-102). 

As shown on the geologic map (PL I, in pocket) the monzonite porphyry of the south 
slope of Prospect Hill is traversed by a nearly east-west dike of quartz monzonite porphyry. 
As previously pointed out, the surface exposures of this dike are obscure, and the actual outcrop 
may not coincide accurately with the mapped outline. The dike may be more variable in 
width and less continuous along its strike than is represented in Plate I. In the underground 
workings the quartz monzonite porphyry has been found only in that part of the Extenuate 
level lying east of the main crosscut and in some of the old, partly accessible workings above 
this part of the level. As is shown in figure 12, the principal body of the quartzose porphyrj' is 
exposed at that part of the level where the Siam and Iron veins approach each other under the 
old Wellington tunnel. The relations of this mass, which may be seen to be of irregular plan 
with little to suggest dikelike continuity, are obscure. In some places it is in igneous contact 
with the monzonite porphyry, but in most places the two rocks are separated by fissures having 
slickensided walls and containing varying quantities of gouge and ore. The mass appears to 
represent an originally irregular dike that has been modified in shape by faulting. In the now 
inaccessible east drifts the dike is said to be more regular, although it is cut into segments that 
are offset by north-south faults. Whether the quartz monzonite porphyry exposed at the face 
of the main Extenuate crosscut (see fig. 12) is continuous with the body just described is not 
known. The Iron vein on the Extenuate level is in quartz monzonite porphyry. About 100 
feet above the level the hanging wall of the vein is monzonite porphyry, and about 90 feet 
above this the whole vein, for the short distance along which it could be examined in 1909, is 
in the monzonite porphyry. No quartz monzonite porphyr}" had been found on the Oro level 
at the time of visit. 

ATTITUDES AND GROUPING OF THE VEINS. 

The veins worked in the Wellington mine are members of a linear system that extends in a 
general N. 37° E. direction diagonally over the southeast slope of Prospect Hill from a point 
in the bed of French Creek, northwest of the mouth of Nigger Gulch, to the west side of Lincoln 
Park, on the northwest side of Mineral Hill. Other mines and prospects on the same group of 




. WELLINGTON MINE FROM THE LOWER COUNTRY BOV TUNNEL. 
¥ the Ofo woikings. the old Wellington tunnel being nearly lialf a nule up the gukl 
I the vein IS diagonally up the hill lo the right, nearly along the line of dumps. Th 
M:||. 3. Main Oto adit. 3. Old Oro adit. 4. Oro shaft. 5. Siam tunnel. See pag 










B. FARNCOMB 


HILL 


FROM THE 


NORTH. 


steep 


idge in the dist 


nee 


sBala 


Mount 


n.s 


parat 


d from Far 


com 


3 Hill by F 


of the 






cho 








om the Q 




shaft hou 




diagonally do«r 


thfl 


nil t^ 






.hlch 


s the sm; 




ne on the 




east of the On 




arks 


he wor 




of the 


Key West 


nd H 




these. 








•ed w.t 










' the Wap 


Key West may be seen 


the old pi 


cer wor 


k.ng 


at the 


head of G 


"•g- 





VEINS OF THE ZINC-LEAD-SILVER-GOLD SEBIES. 



129 



veins are the Old Union, Mono, Minnie, Cincin- 
nati, Lucky, Ella, Tniax, and Melbourne. That 
at least one vein of the group underlies the gravels 
of French Creek has been shown by masses of 
sphalerite scooped from the bedrock by the Re- 
liance dredge. What becomes of the belt south 
of the creek, toward Nigger Hill, is not known. 
Exposures in this direction are poor, and al- 
though numerous fissures traverse the quartzite 
of Nigger Hill, their identity with the Welling- 
ton fissure zone is by no means clear. Knowl- 
edge of the northeast end of the zone is likewise 
meager, the workings of the Melbourne shaft, 
which might throw some light on this question, 
being unfortunately inaccessible. The known 
length of this chain of fissures is about If miles, 
and its greatest width is probably between 1,000 
and 1,500 feet. 

No one fissure is continuous for the whole 
length of the zone. Owing to lack of outcrop 
and to the disconnected character of the work- 
ings, many of which are unmapped and can no 
longer be entered, it is impossible to present a 
plan of the veins as a whole or to describe their 
relations to one another in any but general terms* 
It is reasonably clear, however, that the belt was 
made up originally of a dominant set of nearly 
parallel fissures, each fairly regular along its mid- 
dle portion but pinching out or splitting up in 
various irregular ways at its ends. Crossing 
these parallel fissures are some important east- 
west fissures. The prevailing dip of the princi- 
pal veins is southeast or south at angles ranging 
from 45° to 90**. The most common or average 
dip is about 65°. 

In the Wellington mine the principal veins 
recognized are the Oro, Siam, Orthodox, Spur, 
Iron, and East veins. The Oro vein has been 
followed underground from the vicinity of the 
Oro shaft for a distance of about 600 feet to the 
northeast. As may be seen from figure 12, it 
is not a simple steeply inclined sheet of ore and 
gangue but divides into several branches and 
is offset by cross faults. The details of these 
complications can not be satisfactorily worked 
out from existing exposures, but it appears 
probable that the Oro vein is not really distinct 
from the Siam vein but is linked to the latter 
by ore-bearing fissures. In the old Oro level 
(see fig. 12) the Oro vein is cut off about 550 
feet northeast of the shaft bv a fault fissure that 
strikes about N. 47° E. and dips 60° NW. The 
fissure, which is generally about a foot wide, is 
filled \^ith soft crushed porphyiy containing a 

90047°— No. 75—11 ^9 



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O 

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SB 
d 

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c 

SB 

O 

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130 GEOLOGY AND OBE DEPOSITS OF BBECKENRIDGE DISTBICT, COLORADO. 

little broken sphalerite. The fault movement is apparently normal and offsets what is sup- 
posed to be the northeast continuation of the Oro vein about 75 feet to the southwest. 
Between this old Oro level and the newer workings on the Siam vein at about the same level 
there are no open drifts or crosscuts, and it is impossible to offer more than the suggestion 
that the Oro vein is merely one or more of the branches into which the Siam vein splits as it 
is followed to the southwest. This view is supported by the fact that some of the old stopes 
worked from the Oro shaft were carried up on the Siam vein and connect with the present 
Oro adit. (See fig. 12.) 

The present main adit or Mill tunnel is a crosscut for about 100 feet and then for 450 feet 
in a general northeast direction follows a rather devious fissiure zone containing a few stringers 
of sphalerite and some seams of gouge, but no ore. The general dip of the fissiures along this part 
of the drift is to the northwest at angles up to 85°, the inclination being opposite in direction 
to the normal dip of the Siam vein. 

At the end of the 450-foot stretch of barren Assuring the^ drift reaches an ore body and, leav- 
ing the fissures hitherto followed (see fig. 12), turns sharply from a course of about N. 70** E. 
to one of N. 35° E., wliich is the local strike of the ore. The dip of this ore is 75° NE., and it evi- 
dently is not the main Siam vein, although it probably joins that vein above tliis level. On the 
level a few unimportant stringers lead about N. 70° E. for 75 feet into the Siam vein, which is 
wide at this point and dips 65° SE. The breaks in the continuity of the ore just described are 
apparently not an effect of faulting after the ore was deposited, although seams of gouge testify 
to some such late movement as a minor feature of the structure. They are due essentially to 
the fact that the fissuring had originally a branching character, the spurs showing wide diver- 
gence both in strike and dip from the general attitude of the fissure zone as a whole or of its 
most persistent members. 

The main Siam vein, from the point where it is first entered by the Mill tunnel to a point 
under the Wellington tunnel (see PI. XXVIII), has been followed continuously for a distance 
of over 1,200 feet. Its course, however, is far from straight and the fissuring is not everywhere 
accompanied by ore. Here and there subordinate fissures or spurs make off into the walls of 
the drift. Two of these branch veins, known as the Orthodox vein and the Spur vein, are 
important and contain some ore. 

The Orthodox vein, wliich branches from the Siam vein to the east and possibly to the 
west also, is practically vertical. Its course near the Siam is N. 75° E., but east of the Extenuate 
timnel it strikes generally N. 50° E. and is displaced by faults, as indicated in figure 12. On the 
Oro level the Orthodox vein is not known to cross the Siam vein. On the Extenuate level, how- 
ever, 100 feet above the Oro, a spur vein lying in the general coiuse of the Orthodox has been 
followed for a short distance west of the Siam vein. It consists of a few irregular stringers 
with no ore in conmiercial quantity and has a general dip of about 70° N. It is probably the 
Orthodox vein. 

About 300 feet northeast of the junction of the Orthodox and Siam veins is a short west 
branch from the Siam, known as the Spur vein. Tliis dips 60° S. and carries a good body of 
ore on the Extenuate level. Although the Spur vein is in places 8 feet wide it pinches and 
becomes very small about 150 feet west of the Siam vein. It has not been explored imme- 
diately east of the Siam vein and no indication of an important vein was noted in that wall of 
the drift. A line of fissuring may, however, extend through to what is known as the East 
vein. (See fig. 12.) 

Northeast of the Spur vein the Siam vein maintains its regular course and is continuously 
ore bearing on both levels for a distance of about 350 feet. In the vicinity of the Wellington 
timnel, however, the vein terminates abruptly at an east-west zone of disturbance known in part 
as the Fault vein. On the Extenuate level this ending occiu^ in quartz monzonite porphyry. 
(See fig. 12.) 

Tlie so-called Fault vein consists of one or more strong seams of gouge accompanied by a 
little ore or sulphides and by much polislung and groo\ing of the wall rock. In some places 
the zone of slipping and fracturing is from 30 to 40 feet wide and on tliis accoimt it is diliicult 



U. & GEOLOGICAL SURVEY 
GEORGE OTIS SMITH. DIRECK 



PROFESSIONAL PAPER 75 PLATE XXVIII 




iNu^'AVLf Asc, >« Hiti m. tv'. I scca<r«» (^i. <^j'M> 



s 




TMnl or 246-foot level, 
Oro shaft (under water) 



Fourth or S24-f oot level, 
Oro ahaft (under water) 



VEINS OF THE ZINC-LEAD-SILVEB-GOLD SEKIES. 131 

without more extensive exposures than are now available to outline accurately its course 
and dip. It appears to run in general about N. 65** to 75° E,, or very nearly parallel with the 
Orthodox vein, and dips to the south at angles of 55** to 75°. 

North of the Fault vein and about 50 feet northwest of the point where the Siam vein 
would be if it maintained its original coiirse, is the Iron vein. This strikes in general northeast 
and dips 65° SE. It is thus nearly paraUel with the Siam vein and a glance at figure 12 suggests 
that it is nothing more nor less than an offset part of the Siam vein. The Iron vein was exten- 
sively stoped above the Extenuate level and westward to the fault, but it contains no ore to 
speak of on that level, where it is nearly all in quartz monzonite porphyry. 

There is said to be another vein, the Wellington, lying from 50 to 75 feet southeast of the 
Iron vein in the old Wellington workings. Tliis, however, has not been definitely recognized 
on the Extenuate level and nothing could 'be seen of it in 1909. It may be merely a branch of 
the Iron vein. 

The East vein, as its name implies, lies east of the northeast end of the Siam vein, but'appar- 
ently south of the Fault vein. Its general course as exposed along a distance of about 200 feet 
on the Extenuate level is N. 75° E., or nearly parallel with the Fault vein. Its dip varies 
from 45° to 65°, and tliis accoimts for the change in direction of the vein between the Oro and 
Extenuate levels, as shown in figure 12. Although the East vein has been regarded by some 
as a faulted portion of the Siam vein, this is not probable. It is more Ukely to be a part of 
the Spur vein. 

FAULTING. 

The veins of the Wellington group have been affected by at least two chronologically 
distinct sets of faults. The principal fissures of the older group strike nearly east and west 
and are most conspicuously exemplified by the Fault vein; those of the younger group run 
nearly north and south and are best seen in the workings on the East vein. There are also 
some minor displacements by fissures having various trends and not clearly belonging to either 
of the above groups. 

The Fault vein of the miners, or, as it may better be designated, the Extenuate fault zone, 
is visible on the Oro level, on the Extenuate level, and in the part of the old WelUngton workings 
reached through the Iron raise (fig. 12). On the Oro level, owing mainly to the necessity for 
close lagging of the broken ground, no structural details can be made out. The Siam vein comes 
to an abrupt end and the drift continues for some distance through soft shattered porphyry, 
whence it emerges as it curves southeast to the East vein. On the Extenuate level the con- 
ditions for observation are more favorable. About 50 feet before reaching the fault the Siam 
vein bends westward and quartz monzonite porphyry appears in the hanging wall, as shown in 
figure 12, Thirty feet farther on both walls are composed of the sihcic porphyry, wliich appar- 
ently was offset about this distance by the faulting that attended the opening of the vein. A 
few feet farther on the vein is slightly offset by what appears to be a branch fissure of the main 
fault zone. The ore here shows brecciation and at the junction of the drifts it is crushed, bent 
to the west, and tails off as dragged ore along one of the fissures of the Extenuate fault zone. 
The bend in the Siam vein near the fault and the direction in which the ore is dragged both 
indicate that the continuation of the Siam north of the fault has been displaced to the west. 

Most or aU of the small faults noted in the mine are apparently of the normal type and the 
Extenuate fault zone is probably also one in which the hanging, walls of the fissures have moved 
downward relatively to the foot walls. Inasmuch as the Siam vein dips to the southeast and 
the fissures of the Extenuate fault zone dip to the south, normal faulting with movement 
straight up or down the dip would offset the northeastern continuation of the Siam vein to the 
hanging-wall or east side. The movement in this case, however, was not up or down, but along 
Unes corresponding more nearly to the strike of the fault. 

The walls of the gouge-filled fissures of the Extenuate fault zone are in many places beauti- 
fully polished and are marked by grooves or striations which pitch to the east at angles ranging 



132 GEOLOGY AND ORE DEPOSITS OF BBECKENBIDGE DISTRICT, COLORADO. 

from 10°^ or less, to about 45^} Evidently displacement had a large horizontal component. 
The hangmg wall slipped east much more than it slipped down, and consequently the part of 
the Siam vein north of the fault has probably been offset to the west instead of the east; in 
other words, it would have the relative position of the Iron vein, which is regarded as being 
the continuation of the Siam vein north of the fault. It corresponds closely to the Siam vein 
in dip and strike, is cut off at its southwest end by the same fault zone, lies in the proper position, 
and above the Extenuate level, where it is nearly barren, shows mineralization similar in 
importance and kind to that of the Siam vein. The maximum net movement along the fault 
zone can not be determined accurately owing to the curvature of faults and veins. If, however, 
the dip of the fault zone and of the veins is taken at 65° and the pitch of the direction of prin- 
cipal movement (that is, the angle between the striations and a horizontal line, measured in 
the plane of the fissure) is taken as 15°, then the total net slip between the walls necessary to 
effect the present offset between the Siam and Iron veins would be from 150 to 170 feet. If 
the movement was more nearly vertical than would accord with striations having an average 
pitch of 15°, then the total net slip must have been greater. It may be of interest to note that 
had the movement been such as to grave striae pitching about 80° E., then it would not have 
perceptibly offset the vein, as this is approximately the angle of pitch of the trace of the Siam 
vein on an ideal fault plane with a 65° dip to the south. 

The displacement apparently was not confined to a single fissure and the details are not 
satisfactorily exposed in the workings now accessible. A strong gouge-filled fissure passes 
obUquely ttirough the Iron vein at the raise shown in figure 12. This, however, does not appear 
to be the place of principal movement, for the Iron vein is said to continue southwest past this 
raise to a point witliin 50 feet or so of the main Extenuate crosscut. Here it is definitely cut 
off. Such parts of the old stopes as are accessible through the Iron raise also show the vein to 
continue for some distance past the raise, although it is cut by a number of gouge-filled fissures 
that cross it at smaU angles. Between the Iron raise and the end of the Siam vein on the 
Extenuate level intervenes about 30 feet of fissured and disturbed quartz monzonite porphyry, 
which is probably within the fault zone. 

/ By cUmbing about 100 feet up the Iron raise it was possible in 1909 to penetrate southwest 
through the old stopes on the Iron vein to the Extenuate fault zone, where the vein abuts 
sharply but obliquely against a strong seam of gouge. At this place the Iron vein strikes 
northeast and dips 65° SE. The principal gouge seam strikes N. 65° E. and dips southeast 
at the same angle as the vein. The zone of gouge and crushed porphyr}' exposed at this point 
is fuUy 10 feet wide. 

The Extenuate fault zone is not altogether younger than the Siam vein. Along it in 
places are stringers of sulphides and ore enough to have encouraged considerable prospecting. 
Some of the ore was imdoubtedly dragged in from the Siam vein, but some appears to have 
been originally deposited along the fissures. It is probable that the fault zone was in the 
first place an east-west cross vein like the Orthodox and was formed and filled at about the 
same time as the Siam vein. Later stresses transformed this vein into the Extenuate fault 
zone and separated what is known as the Iron vein from the Siam vein, with which it was once 
directly continuous. 

A series of north-south faults younger than the Extenuate fault zone have further com- 
plicated the structure of the northeastern part of the mine. One of the best exposed of these 
is the fault that cuts off the East vein at its west end on the Extenuate level. This fissure 
dips 85° W. and contains from 1 to 2 feet of crushed porphyry and soft gouge lying between 
slickensided walls with nearly vertical striae. The hanging wall, not well exposed, is probably 
quartz monzonite porphyry; the footwall is monzonite porphyry. This may be the same 
fissure that displaces the Orthodox vein just east of the main adit crosscut, but lagging at tliis 
place conceals the course of the chief disturbing fissure. 

I since the observations on which this description \a based were made, Mr. John Wellington Finch has completed a careful structural study of 
the inlne, and his report, which he very kindly sent to me, has helped materially to clear up some doubtful points regarding the relation between the 
Slam and Iron veins Mr. Finch distinguishes two sets of these striations— an older set pitching at angles from 10* to 20" and a yoimger set 
pitching from 40* to 50". If he is correct in this It is possible that close observation might somewhere detect traces of the older grooves directly 
crossed by the younger.— F. L. R. 



VEINS OF THE ZINOLEAD-SILVER-GOLD SEEIES. 133 

About 100 feet east of the cross fault just described the East veiii suffers a second dislo- 
cation, as shown in figure 12. This fault fissure is not well exposed but apparently dips 60° E.* 
A number of other north-south faults are reported also in the generally inaccessible workings 
east of those shown in figure 12. 

West of the Extenuate tunnel the Orthodox vein, although crossed by a few faults of small 
displacement, is fairly regular. East of the tunnel, however, the entire drift supposed to be 
on the Orthodox vein is in soft, very much disturbed rock in which the vein can be followed 
only as a series of crushed fragments. The numerous gouge-filled sUps for the most part show 
no regularity in arrangement or direction and appear to have little individual persistency; 
but better exposures would probably show the vein to be cut by northnsouth faults much as 
is the East vein. Doubtless some of these fissures continue through from one vein to the other. 
At about 200 feet from the Extenuate tunnel the vein is displaced by a strong gouge-filled 
fissure with a dip to the east of only 15°. The general disturbance in the neighborhood of this 
fault, however, is so great as to obscure its effect on the vein and the direction and character 
of the displacement were not ascertained. 

Several faults of small to moderate displacement are exposed in the old Oro tunnel in the 
western part of the mine. The most important of these has already been described on page 129. 

The movement along the north-south faults was probably accompanied by some renewal 
of movement along the east-west faults. It is to tlus later movement that Mr. Finch ascribes 
the steeper set of stria) noted by him on the walls of the Extenuate fault zone. 

CHARACTER AND MATERIALS OF THE VEINS. 

The veins of the WelUngton mine attain a maximum width of about 15 feet, most of the 
stopes ranging from 4 to 10 feet. Those over 8 feet wide generally include one or more slabs 
or horses of country rock. The vein roaterial is composed essentially of pyrite, sphalerite, 
and galena in various proportions with relatively little gangue. Such waste as occurs within 
the ore bodies consists as a rule of horses or small fragments of metallized porphyry or of pyrite 
containing too small a proportion of galena and sphalerite to be classed as ore. True gangue, 
where present, is siderite or barite. These minerals, however, are nowhere abundant and axe 
younger than the sulphides, in which they occur as veinlets or as the lining of vugs. 

Metasomatic replacement has played an important part in the formation of the veins, 
but the process, so far as relates to commercial ore bodies, has been confined to fissured or 
crushed rock. Consequently the ores do not extend irregularly into the country rock on each 
side of the vein, but lie as a rule between fairly well-defined walls, which correspond closely to 
the original boundaries of the main fissures. Fragments of porphyry within the fissure zone, 
especially if of small size or traversed by many cracks, have been more or less changed to 
ore, partly by the filling of interstices and open spaces, partly by replacement. The ore is gen- 
erally solid and firm and is accompanied by no persistent gouge along either wall. Locally 
gouge may be present between walls and ore, and the ore bodies in a few places are traversed 
by fissures filled with crushed ore or gouge. In the vicinity of faults, also, the ore is fissured 
and broken. 

Although the vein material is generally firm, it is not entirely solid, small interstitial spaces 
between the crystals, or vugs, some of them incrusted with siderite or barite, being so abundant 
as to give the whole a porous texture. The constituent sulphides are not arranged in any 
general order, and the ore affords Uttle more than a suggestion of original banding here and there. 
The prevailing texture is that of a granular aggregate of galena, sphalerite, and pyrite, the three 
being combined in different proportions in different places and showing much variation in 
coarseness of crystallization. Crystals of galena attain the largest size, some of them being 
2 inches across. This, however, is exceptional. 

A very characteristic feature of the Wellington ore is the manner in which it is traversed 
by white or pale buff veinlets of carbonates approximating to siderite in composition. In some 

1 This fault, according to Mr. Finch, has been cut also on the Oro level in the east drift on the East vein, extended since my own visits to 
the mine.— F. L. R. 



134 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

parts of the veins these veinlets, most of which are less than an inch in width and are inclined 
to be vuggy along their middle planes, are arranged parallel with the walls of the main vein, 
giving the ore a banded appearance. In other places the carbonate veinlets branch irregularly 
through the sulphides in all directions, and in a few places the sulphides have been brecciated 
and the fragments are now cemented by the sideritic carbonate. The carbonate veinlets are 
distinctly later than the principal epoch of sulphide deposition and as a rule contain no sulphides 
except in the form of included fragments. The siderite is present in some form in nearly all of 
the ore, either as the veinlets just described or as the lining or filling of small interstices in the 
original porous bodies of sulphides. 

The tenor of the Wellington ore varies widely, some being essentially a lead-silver ore and 
some essentially a zinc ore. A typical technical analysis of the crude shipping ore is given in 
the table on page 108. Another variety of the ore, such as that in the east face of theOro level 
in 1909, carries 38 per cent of zinc with only 1 per cent of lead. Technical analyses of the first- 
class concentrates and of carbonate ore from surface workings on the Wellington ground are 
also given on page 108. 

OXIDATION OF THE ORE. 

Even along the outcrops of the veins most of the shafts and tunnels show some galena in the 
claylike product resulting from thorough oxidation, and the change to essentially sulphide ore 
generally takes place at depths of less than 300 feet. The depth of the oxidized zone, however, 
varies, being greatest in general near the crest of the ridge in which the ore bodies occur and 
least along the lower slopes. On the main levels of the Wellington mme the ore as a rule shows 
no oxidation, although in a few places there has been a slight development of smithsonite in 
interstices of sphaleritic ore. Owing to the inaccessibility of the old upper workings there is 
little opportunity at present for studying the transition from partly oxidized ore to sulphide 
ore. The normal sequence, however, from the surface down, appears to be ( 1) a soft, heavy, 
yellowish clay-like ore consisting largely of earthy cenisite and containing residual nodules of 
galena; (2) a lead-silver ore in which the galena is only in part oxidized, while the pyrite has 
been for the most part changed to limonite and the sphalerite altered to smithsonite and limonite, 
with removal of much of the zinc in solution ; and finally ( 3) a lead-silver-zinc ore in which galena 
predominates and in which the early stages of oxidation are indicated by the formation of a 
little spongy smithsonite, or " drybone," as the miners call it, at the expense of the zinc blende. 

COXJNTBY BOY MINE. 
INTRODUCTION. 

The Country Boy mine is about a quarter of a mile south of the Wellington, on the opposite 
side of French Creek. It has been intermittently productive since about 1889, first of a silver- 
lead ore carrying some gold and afterward of a high-grade zinc ore. The property has long been 
in litigation and consequently its development has been neither steady nor systematic. In 1907 
the mine is reported to have yielded 76 cars of ore carrying from 47 to 55 per cent of zinc. It 
was idle in 1908 but was being worked again in 1909, producing a zinc ore that was shipped as 
it came from the stopes. Very little development work, however, was in progress. 

UNDERGROUND WORKINGS. 

It was found impossible to procure any complete map of the Country Boy mine, but a general 
plan and section of a part of the underground workings of the mine are shown in figure 13. 
There are two adits 170 feet apart vertically and not connected. The upper tunnel runs about 
S. 18*^ E. for 700 feet and then, turning slightly, continues for 1,000 feet in a S. 25° E. direction. 
This tunnel cuts the Country Boy vein between 400 and 450 feet from the portal, and is con- 
nected with old drifts and stopes on this vein, now only in small part accessible. About 350 
feet beyond the Country Boy vein the tunnel cuts the Hite vein, which was being prospected in 
1909 by a winze. The lower tunnel runs southeast for 1,050 feet to the Country Bo}^ vein, 
where it connects with short drifts and witli stopes about 250 feet in total length and 90 feet 
high at the time of visit. 



VEINS OF THE ZINC-LEAD-SIL.VER-GOLD SERIES. 



135 



GEOLOGIC RELATIONS. 

The rocks are poorly exposed in the vicmity of the mine. The principal rock of Nigger Hill 
is monzonite porphyry, which is part of the same great irregular sill that forms the crest of Bald 
Mountain. Both east and west of the Country Boy mine this porphyry comes down to the bot- 
tom of French Oulch. (See PI. I, in pocket.) In the immediate vicinity of the mine, however, 
is a triangular area of black shale and quartzite inclosing an oval area of monzonite porphyry. 
Presumably the shale overUes the quartzite, and both are cut by the porphyry, but these relations 
are not evident from what may be seen at the surface. Another possibility, which could not be 
verified, is that the vein occupies a fault fissure which at the surface separates quartzite on the 
southeast from porphyry and shale on the northwest. 

The upper tunnel, at an elevation of 10,100 feet, enters the hillside in the oval area of 
porphyry and continues in this rock for 250 feet. It then goes through black shale for 37 feet, 
but as both rocks are here greatly disturbed it is impossible to say what their stuctural rela- 
tions are at this place. Porphyry, with a few masses of much-broken black shale, continues to the 
Country Boy vein, 450 feet from the portal. Beyond the vein the tunnel passes through a little 
shattered quartzite, of which the stratigraphic relations are very obscure, into gray shale that 
dips about 15° N. and apparently underlies the quartzite. At a distance of 600 feet from the 



UPPCR TUNNEL 



LOWER TUNNEL 




o 



too 



♦00 



eooFoet 



UPPER TUNNEL 



LOWER TUNNEL 




muimmmm'* ♦#    ♦..-^ii.i. ^ .i 




Fault and Hit* vain 

Fault and Country Boy vain 
Paul 



  • 

4   • 

    

Monzonita  porphyry « 

   

 ♦♦♦44 44 






 4  4 

 ♦♦♦■•♦♦^ 

Monzonita porphyry ■» I 

:."  4     4J 

^' 4 4 4  1. 



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FiGUBE 13.— Plan and section of the Country Boy mine. Only a part of workings shown. Geologic section is largely hypothetical. 

portal the tunnel passes from this shale into the upper part of a porphyry sill which it trav- 
erses for about 425 feet. Both upper and lower surfaces of the siU, where cut by the tunnel, 
are regular and conform to the bedding, which dips 17° NNW. These relations indicate an 
actual thickness of about 100 feet for the intrusive sheet. There is no appreciable contact 
metamorphism of the beds. 

After emei^ng from the under side of this sill, which is probably connected directly with 
the irregular sheet of porphyry forxning the upper parts of Nigger Hill and Bald Mountain, 
the tunnel continues for nearly 700 feet in greenish-gray calcareous shale. This dips generally 
yjo j^NW. but at the face has a dip to the east of only 5°. The shale is cut by four or five 
faults of small to moderate displacement. They strike generally from northeast to north- 
northeast and dip southeast. The throw for the smaller faults, amounting in one fault to 18 
inches, is normal, and probably all are of this type. 

The stratigraphic position of this gray shale is not entirely clear. It is thought to underlie 
the Dakota or to be a basal phase of that formation. 

The lower tunnel, at an elevation of about 9,930 feet, goes through black calcareous Upper 
Cretaceous shale for about 500 feet. Near the portal this strikes N. 60° W. and dips 25° NE. 
Near the 500-foot point it strikes N. 85° W. and dips 50° N. One small sill of porphyry occurs 



186 



GEOIJJGT AND QBE DEPOSITS OF BBECKENBIDGE DISTBICT, COLOBADO. 



in this shale abbut 450 feet from the portal. For about 300 feet beyond this shale the tunnel 
goes through a porphyry sill which is cut by some faults and contains a small mass of black shale 
apparently faulted in. Beyond and under the sill is more black calcareous shale striking N. 60° 
J to 70° W, and dipping from 30° to 45° NE. The tunnel traverses this 

^ s shale for 125 feet and then passes through a fault fissure into diorite por- 

phyry, which contains the Country Boy vein on this level. The fault strikes 
N. 10° E., is practically vertical, and probably belongs to the same class as 
the north-south faults of the Wellington mine. 

The ascertained facts regarding the geology of the Country Boy mine 
and a tentative interpretation of the structure are shown in figure 13. The 
data obtainable, however, are so meager and unsatisfactory that the dia- 
gram is rather a suggestion of possibilities than a representation of known 
relations. It is not intended for an accurate section through the mine. 



f,«V*- 



i' 



I 



ill % 






The Country Boy vein, owing to the condition of the old workings, 
could be studied in 1909 only in the stopes above the lower tunnel. Its 
■s general strike is approximately northeast and it dips northwest, apparently 
I at an average angle of about 80°. The part of the vein examined is entirely 
I in monzonite porphyry and attains a maximum width of about 5 feet. 
3 The beat ore consists of rather friable dark sphalerite with practically 

g no gangue except a fine granular sideritic carbonate that is younger than 
% most of the sphalerite and fills fissures and interstices in the ore. At the 
I time of visit the stope was about 250 feet long and at the ends showed a 
J gradation from the high-grade sphaleritic ore into material containing 
I abundant pyrite. Galena is rare in this stope and occurs only in small 
-| bunches up to the size of a man's fist. 

J Some of the sphalerite, occupies clean-cut fissures in the porphyry, but 

I, most of the ore appears to have filled interstices and replaced crushed por- 
^ phyry in a zone of fissuring and brecciation. WhUe the boundary between 
I ore and country rock is generally definite, the neighboring porphyry shows 
^ considerable metasomatic alteration and contains pyrite and sphalerite in 
veinleta and in disseminated crystals. 

The ore has not been much disturbed since its original deposition, but 
there appears to have been some movement along the plane of the vein after 
the sulphides were formed, and where the ore pinches the course of the vein 
b marked by a seam of tough gouge. It is by no means certain, however, 
that this gouge is entirely younger than the ore. 

The unsorted ore as mined in 1909 was said to carry from 42 to 43 per 
cent of zinc. No attempt was being made to work milling ore. 

HELEN MINE. 

The Helen mine lies on the south side of French Gulch between the 
Country Boy mine and the mouth of Australia Gulch. The old workings, 
^' principally a tunnel that enters in the quartzite spur just west of Australia 

Gulch (see PI. I, in pocket), are closed by caving. Recently a lower tunnel has been run from 
a point 800 feet northwest of the old tunnel an<l 200 feet lower. The new tunnel has a course 
of S. 20° E. and is about 927 feet long. It was examined during the preliminary visit to the 
district in 1908, but in 1909 work had been abandoned and the tunnel was blocked by a fall of 
Bhole. 



VEINS OF THE ZINC-LEAD-SILVER-GOLD SERIES. 137 

The geologic section afforded by the Helen tunnel is similar to that of the Country Boy 
Tunnels and is shown in figure 14. Shales, including Upper Cretaceous shale and gray cal- 
careous* shale of the Dakota, have been intruded by sills and dikes of diorite porphyry and 
all the rocks have been subsequently faulted. 

The vein, which is cut at about 876 feet from the portal of the tunnel, strikes N. 60° E. 
and dips about 66** SE. The walls, so far as exposed in the workings, are monzonite porphyry. 
The vein material is very similar to that of the Country Boy, but the stringers of sphalerite 
thus far opened in the Helen tunnel are too small to mine with profit. 

SALIJE BABBEB MINE. 

The Sallie Barber mine is situated on the steep spur east of Australia Gulch and south 
of French Creek. It is worked through a shaft 365 feet deep, with levels at 240, 300, and 350 
feet. The lowest level at the time of visit was about 100 feet long, and neither of the upper 
levels exceeds 300 feet in length. The mine is accordingly a small one and has no great 
production to its credit. 

The Sallie Barber vein strikes N. 54 ** E. and dips 80^ NW. It is entirely in monzonite 
porphyry and is a fairly regular zone of crushed metallized rock, in places 9 feet wide. The 
strike of the vein carries it obliquely across the Sallie Barber claim for about 600 feet, the ground 
beyond, on both sides, being part of the Little Sallie Barber property. 

The ore, consisting of sphalerite and pyrite, much of it very friable, occurs partly in bunches 
and stringers filling original spaces in the fissure zone but mainly as a metasomatic replace- 
ment of crushed porphyry. The vein contains comparatively little gangue. The most abun- 
dant mineral of this class is a sideritic carbonate, which, as in the Wellington and Country Boy 
mines, is younger than most of the sphalerite and pyrite. Its deposition was accompanied, 
however, by the formation of considerable younger pyrite and of a little sphalerite, the latter 
being rosin-colored whereas the older sphalerite is nearly black. 

Oxidation of the pyrite extends to a depth of 250 feet. From the surface down to 200 
feet no ore of consequence was found. Below that and above the 240-foot level was stoped a 
lead ore consisting of cerusite with some residual galena. This ore was foimd to change down- 
ward into the present zinc ore, which is practically free from galena. The ore on the bottom 
level is not entirely free from oxidation, as is shown by the occurrence of some impure smith- 
sonite with the sphalerite. The alteration results in a porous structure, space formerly filled 
by sphalerite or perhaps by gangue carbonates being now occupied by spongy masses of smith- 
sonite, as shown in Plate XXVI, A (p. 126). The pyrite in the ore is unattacked at this stage 
of alteration; indeed, some small crystals of pyrite have been deposited with the smithsonite. 

It appears probable that acid sulphate solutions from the overlying zone of general oxidar 
tion attacked simultaneously the sphalerite and the bunches of dolomitic gangue in the ore, 
setting free carbonic acid, which reacted with some of the zinc sulphate to form smithsonite. 

The ore from the Sallie Barber mine is sorted to a product containing from 30 to 35 per 
cent of zinc and is shipped to the Western Chemical Co. at Denver. 

LITTLE SALLIE BABBEB MINE. 

The Little Sallie Barber shaft is about 300 feet northeast of the Sallie Barber and is 300 
feet deep, with short levels at 200, 250, and 300 feet on what is apparently the Sallie Barber 
vein. Up to the time of visit in 1909 less ore had been found in this part of the vein than in 
the neighboring mine, and no shipments had been made A considerable part of the lode in 
the Little Sallie Barber is a zone of soft crushed porphyry, containing only small bunches of 
sulphides. Some galena was found on the 200-foot and 250-foot levels, but this mineral has 
not been seen on the 300-foot level. Some of the ore shows veinlets and crusts of white calcite 
that are younger than tlie pinkish sideritic gangue common in the sphaleritic ores of French 
Gulch. 



OEOLOOY AMD OBE DEPOSITS OF BBECKENRIDOE DISTRICT, COLORADO. 

Recent reports from Breckenridge indicate that in the spring of 
1910 the American Zinc Extraction Co., owner of tlie Little Sallie 
Barber, was shipping about 30 tons of zinc ore a day, 

FBBNCH CBEEE TUNNEL. 

The French Creek tunnel is on the soutli side of French Gulch, 
due south of Mineral Hill. It runs for 2,100 feet S. 12" E. under the 
porphyry sheet of Bald ^lountain and is of interest chiefly from the 
section that it affords of the sedimentary beds under that poqihyry, 
A longitudinal section through the tunnel is shown in figure 15, and 
the stratigraphy is described on page 71. \o productive ore body 
has been opened in the tunnel. 

FUZZI^, OTTSAT, AND GOIJ} DTTST lONES. 

The Puzzle, Ouray, and Clold Dust workings, situated about 1} 
miles soittlieast of Breckenridge, between Little Mountain and the 
head of Dry Gulch, are all so closely related as to be most conven- 
iently described together. 

The first shipment from the Gold Dust was made late in 1885, 
but the Puzzle and Ouray mines were a few years later in becoming 
productive. In 1890 the Puzzle and Ouray both shipped steadily 
from the same vein and unfortunately became involved in a l^al 
quarrel that hampered their development for over seven years. About 
the year 1 897 the ore began to show signs of exhaustion and although 
the Puzzle and Gold Dust were worked by lessees for a few years 
more all three of the mines had been long abandoned to ruin by the 
opening of 1909. During the summer of that year the Puzzle and 
Gold Dust workings were in part reopened under an option, with a 
view to thorough testing of the vein below the shallow depth to which 
work had been confined. The total production of the three mines 
could not be definitely ascertained. Local estimates, apparently 
reliable, make the gross yield of the Puzzle and Ouray about S960,000 
and of the Gold Dust about $200,000. 

The connected underground workings of the three mines are 
fairly exten.<five but not deep. The principal level of the Puzzle and 
Gold Dust, known as the Willard level, is shown in figure 16, which 
is for the most part a rough survey effected by pacing distances and 
determining directions with a compass. This level has its portal only 
a few feet above the little marshy Hat just east of Little Mountain 
and runs nearly southeast for 715 feet as a crosscut to the Puzzle vein. 
Southwest of the point wliere the crosscut reaches the vein are the 
principal workings of tlio Ouray mine, now abandoned and inaccessi- 
ble. The Ourny shaft, situated on the hillsi<le southeast of the portal 
of the Willard tunnel, is said to be 368 feet deep and to extend 268 
feet below the Willard level. From this level extensive stopes were 
opened on the Puzzle or Ouray vein southwest of the point whore it 
crosses the side line of the Puzzle claim. No map of these workings 
could be found in 1909. 

From the Puzzle side line the Willard level turns northeast and 
follows the vein for 900 feet to the Puzzle Extension shaft, 187 feet 
deep, situated in Illinois Gulch just above the mouth of Dry Gulch. 
Along nearly all of this distance the vein has been stoped to the sur- 
face. At the Puzzle Extension sJiaft the level leaves the vein, which 
here appears to split into unimportant branches, and crosscuts north 



VEINS OF THE ZINC-LEAD-SILVER-GOLD SERIES. 



139 



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140 OEOLOGT AND OBE DEPOSITS OF BBECKENBIDGE DISTBICT. COLORADO. 

for 300 feet to the nearly parallel Gold Dust vein, whose course on the surface coincides with 
the bottom of Dry Gulch. This vein is exploreti on the Willard level for approximately 800 
feet and for much of this distance has been stoped. There are several old levels above the 
Willard tunnel on both veins, but these were not open in 1909. 

The rock exposed along the outcrops of the Puzzle and Gold Dust veins is typical Dakota 

quartzite, just north of which, in Dry Gulch, is a strip of dark Upper Cretaceous shale, which 

supposedly is faulted down against the quartzite, although the fault fiasure is not visible. On 

the Willard level the hard white quartzite is found to be associated in an inseparable way with 

beds of gray shale, in part calcareous. These two rocks in some parts of the mine alternate in 

thin beds, as may be well seen in the crosscut southeast of the Puzzle Extension shaft. The 

general dip of the beds is northwest at 20" to 25°, although locally they show much irregular 

« disturbance. Both shale and quartzite 

sS t ' .? are considered to belong to the Dakota 

^jc ^ Jo i formaticin. Between the Puzzle and 

Qj £ 5 o a Gold Dust veins, as is shown in figure 

16, is a body of diorite porphyry. This 
is an intrusive sheet or sill, the bottom 
of wliich is exposed at the beginning of 
the Gold Dust crosscut, near the Puzzle 
Extension shaft. About 50 feet north of 
the shaft a fault of small throw brings 
the shale beneath the sill up to the level 
of the crosscut, but a gentle northwest 
dip of 20° soon carries it out of sight 
and the rest of the crosscut is all In 
porphyry, which also forms the southeast 
wall of the Gold Dust vein for about 300 
feet. 

There is little opportunity for study- 
ing the veins themselves in the present 
workings, for most of the drifts are through or under old stopes and afford no typical exposures 
of ore and very few of the wall rocks. Both veins on tlie whole are practically vertical and 
appear to occupy fault fissures of small displacement. There was no opportunity at the time of 
visit to measure the throw along either fissure, but it ia rather doubtful if it anywhere exceeds 
100 feet and probably it is considerably less than that. As may he seen from figure 16, the 
Puzzle vein for at least 600 feet of its length lies at or close to the contact between porphyry 
on the nortliwest and quartzite and shale on the southeast. Porphyry and sediments along this 
stretch of the vein were evidently faulted together at tlie time the fissure now occupied by the 
vein was formed. Yet the line of this fault, and of the vein also, differs so little from tlie line 
that would be defined by the intersection of the bottom of the porphyry sill with the horizontal 
plane of the level as to show that no extensive displacement has occurred. The absence of 
any strong continuation of the Puzzle vein east of the Puzzle Intension shaft also points to 
the same conclusion. 

At no place could any ore be seen in the Puzzle vein in 1909. According to Mr. John 
Nelson, who at one time was foreman in the Puzzle mine, all of the rich shipping ore was stoped 
from levels above the Willard tunnel. The stopes immediately above that tunnel supplied an 
unoxidized milling ore averaging about S12 a ton. Most of the shipping ore was sent to Denver, 
but the composition of one lot that went through the local sampling works is given in the table 
on page 108. The shipping ore from the Gold Dust vein was similar, as may be seen from the 
same table. 

In 1909 a small stope was being worked on the Gold Dust vein, about 75 feet above the 
first raise shown east of the fault in figure 16. The best ore, consisting of shattered, recemented 
shale and quartzite carrj-ing pyrite, sphalerite, and a httle galena, shows considerable disturbance  




VEINS OF THE ZINC-LEAD-SILVER-GOLD SERIES. 141 

that has to some extent obscured the original relation of the ore to the country rocks. At this 
place a little soft altered porphyry carrying bunches of sphalerite^ probably a dike, lies on the 
south side of the ore. The general relations of the ore to the rocks in this stope are indicated 
in figure 17. 

WASHINQTON MINE. 

The Washington mine is situated IJ miles southeast of Breckenridge, on the slope between 
Illinois and Dry gulches. This is one of the older mines of the district, and as early as 1883 
was giving employment to over 30 men. A 20-stamp mill was built about 1885. At this 
time and for a few years later the mine was worked through the Watson shaft, situated at an 
elevation of 10,550 feet on the spur connecting Nigger Hill with Bald Mountain. The mine 
continued steadily productive until 1891, after which it appears to have been turned over to 
lessees, who made shipments up to 1897 and perhaps later. The total output of the Washington 
mine, as given in a prospectus issued by the Washington-Joliet Mining & Milling Co., capitalized 
at $1,500,000, is between $400,000 and $500,000. There is no reason to suppose that this 
statement is exaggerated. 

The underground workings consist of the old shaft on the hill, from which considerable 
drifting and stoping was done, and of six tunnels on the slope from the shaft down to Illinois 
Gulch. Of these the most important is the Cornish Tunnel, which follows the Washington 
vein in a general northeast direction for about 1,400 feet and connects with the shaft at a depth 
of 250 feet. At 115 feet below the Cornish tunnel is the Berlin tunnel, supposed to be on 
the Emmet, a vein parallel with and a short distance southeast of the Washington. From 
600 to 700 feet southeast of these tunnels are the Christensen, Mayo, and Weinland tunnels 
on the Mayo vein, which has produced ore to the value of at least $50,000. The Washington 
and Emmet veins are reported to dip northwest at 65° to 70°, but the Mayo, approximately 
parallel with them in strike, dips southeast. From 250 to 300 feet southeast of the Mayo 
vein is another vein striking N. 25° E. and dipping northwest at 75°. This is developed in 
the Horn tunnel. 

All the Washington workings, of which, so far as known, no complete map exists, were 
inaccessible in 1909 and nothing could be learned at first hand of the character of the veins or 
of the relations of the rocks that they traversed. This is especially regrettable, as the long 
Cornish tunnel, which begins in quartzite and connects with the shaft sunk in monzonite 
porphyry and which probably crosses a fault between these two rocks (see PI. I, in pocket), 
ought to furnish an interesting underground section that would throw considerable light on the 
structure of the district. 

The ore of the Washington appears to have been not unlike some of the Puzzle and Grold 
Dust ore, carrying silver and lead with some gold. The composition of one lot shipped from 
the Horn tunnel, and therefore not from the main Washington vein, is given on page 108. This 
evidently was partly oxidized ore. 

JUMBO MINE. 

The Jumbo and Buffalo claims were first developed in the summer of 1884 by E. C. Moody, 
who, in August of that year, ha<l a shaft 50 feet deep exposing a vein of rich gold ore in porphyr}\ 
In the following year he sold his claims for $25,000 and the Jumbo mine entered upon a short 
period of steady production, the output toward the end of 1885 amounting to 35 tons a day, 
which was treated at the Eureka mill, near the mouth of Cucumber Gulch. In 1888 the mine 
was closed, but it was reopened in 1890 and is known to have been producing in 1893 and 
again in 1897. In 1898 it was worked by lessees and shortly after appears to have been 
abandoned to decay. Its total production can not be learned but probably exceeded $300,000. 
The ore is reported to have averaged from $8 to $9 a ton down to the bottom of the oxidized 
zone, where work stopped. Some of the concentrates (although for what year is not known) 
contained 0.46 ounce of gold and 1.9 ounces of silver to the ton, wliich would indicate an ore 
of considerably lower grade than that reported above. 



142 GEOLOGY AND ORE DEPOSITS OF BBEOKENBIDQB DISTRICT, COLORADO. 

The Jiinibo vein is mainly in the narrow strip of diorite porphyry that, as shown on Plate I 
(in pocket), separates the quartzite of Gibson Hill from the shale forming the gentle slope 
southw^est of Gold Run. The vein strikes nearly N. 60° E. and lies very close to the quartzite; 
in fact, a small spur vein of exceptionally rich ore is said to have extended into the quartzite. 
Material on the dump indicates that the deposit is a stringer lode, the fissures being filled with 
pyrite almost free from gangue. Oxidation of the pyrite produced a free-milling ore containing 
some coarse gold. Apparently the pyritic ore below the zone of partial oxidation proved to 
be of too low grade for profitable working. 

EXTENSION MINE. 

The Extension workings, formerly owned by the Double Extension Gold Mining CJo., lie 
just east of the Jumbo workings, and, like them, are too dilapidated to permit examination. 
The deposit was discovered about the same time as the Jumbo but was worked to a later date, 
the company treating the ore in a 20-stamp mill at the mine. The Extension was one of the 
prominent mines in the district in 1892 and 1893 and was supplying $25 ore from a stope up 
to 20 feet wide. The present owner bought the property at a delinquent-tax sale and could 
furnish no information regarding the underground workings. The ore apparently occurs in 
quartzite and so far as is known was not worked below the limit of oxidation. 

LITTLE COBPOBAL MINE. 

The Little Corporal mine is in quartzite just west of the Jumbo workings. The vein, which 
afforded a gold-silver ore, was worked from a shaft which has long been abandoned. Nothing 
could be learned concerning the character of the deposit. 



CHAPTER X. 
STOCKWORKS AND VEINS OF THE GOLD-SILVER-LEAD SERIES. 

DISTRIBUTION. 

The ore deposits of the gold-silver-lead series are all in the northeastern half of the district 
and are as characteristically associated with the quartz monzonite porphyry as are the veins 
described in the last chapter with the monzonite porphyry. The mines in ore bodies belonging 
to this group are the Wire Patch, on the south slope of Famcomb Hill; the Cashier, in Browns 
Gulch, and the near-by I. X. L., on the Swan; the Hamilton, in Summit Gulch; and the Jessie, 
on the northeast side of Gold Run. None of these mines is now in a condition to pennit thorough 
examination, and the Hamilton was the only one where any work whatever was in progress in 
the summer of 1909. 

GENERAL STRUCTURAL FEATURES. 

A distinctive feature common to these deposits is the occurrence of the ore in much-fissured 
and minutely veined rock, ordinarily quartz monzonite porphyry, rather than in well-defined 
lodes. The Assuring may vary widely in character. In the Hamilton ore bodies it is concen- 
trated along neariy parallel zones, so that the pay shoots have a certain regularity and might be 
classed as stringer lodes having widths up to about 15 feet and being separated from each other 
by 50 feet or less of relatively barren porphyry. The ore as a whole, however, has no definite 
walls. The adjacent porphyry is also more or less fissured and contains many smaU stringers 
of sulphides. 

In the Jessie n\ine there is not the same general agreement in direction of the fissures. 
Certain groups of them are approximately parallel, but these are associated with others having 
decidedly different strikes. In many places one set joins or crosses another without any general 
displacement of either. As in the Hamilton mine, however, there is a decided tendency of the 
fissures toward a lodelike grouping, as may be seen from figure 18 (p. 145). At the Jessie mine 
a mass of porphyry, oval in plan, fully 900 feet long, 600 feet wide, and 300 feet deep, has been 
fissured in many directions, but especially by fractures striking from east to northeast. The 
whole is thus a low-grade stockwork; but mining has in the past been confined to those zones in 
which the fissures are closely spaced and most of the stopes are such as might have been opened 
on ordinary veins of moderate width. As may be inferred from figure 18 tliere is generally no 
great persistency to these lodehke zones of stringers. Some die out within surprisingly short 
distances or merge with other zones. 

In the Cashier mine the principal ore body appears to have been a rotund mass of veined 
porphyry, which was stoped as a whole. Details of the fissuring are not now discernible in this 

mine. 

The ore body of the I. X. L. mine as visible in the lower tunnel is a shattered mass of quartzite 
intricately intruded by quartz monzonite porphyry, the fissures and interstices in the mass 
being partly filled with sulphides. Inasmuch as the country rock is largely quartzite this deposit 
is regarded as less typical of the group than the others here described. The Wire Patch ore 
bodies are in some respects intermediate between those of the I. X. L. and Casliier mines, as they 
occur in irregularly fissured quartz monzonite porphyry and also in porphyry crowded with 

fragments of sedimentary rock — in this case shale. 

143 



144 GEOLOGY AND 0B£ DEPOSITS OF BBECKENBIDGE DISTBICT, COLOBADO. 

THE ORES. 

The deposits of this group yield generally a low-grade pyritic ore, concentrated principally 
for its gold and silver contents. There may be or may not be enough lead present to add to the 
value of the product. . Mineralogically they consist of pyrite, sphalerite, and galena in a gangue 
of sericitized porphyry. Report indicates that the oxidized and partly oxidized upper portions 
of these deposits were considerably richer in the precious metals and in lead than the deeper 
sulphides, and it is known that bunches of rich ore containing galena and native gold were found 
in some of the upper workings. The greater part of the sulphides fills small open fissures (see 
PL XXVI, -B, p. 126), but the veining is associated with some replacement. In the Jessie mine 
this metasomatism does not result in any general replacement of the porphyry by sulphides, so 
far as observed, but pyrite, sphalerite, and galena all develop sporadically at the expense of the 
orthoclase phenocrysts. In the Wire Patch ore bodies, however, the sulphides appear to have 
replaced the porphyry bodily to some extent. 

EXAMPLES AND DETAILED DESCRIPTIONS. 

JESSIE MINE. 

INTBODUCTION. 

The Jessie mine (see PL XXIX, A) is situated on the northeast side of Gold Run, about 
2 J miles northeast of Breckenridge. The group comprises 44 patented claims, covering a 200- 
acre strip of ground that extends from Gold Run to Galena Gulch. The productive part of 
the group, including the Jessie, May B., Baden Baden, and Berlin claims, Ues on the slope 
overlooking Gold Run. Work on these claims appears to have begun in 1885 by E. C. Moody,* 
a prospector who had previously been successful on Famcomb and Gibson hills. In 1886 he 
and his partner, Irwin, had a 10-stamp mill on the site of the present Jessie mUl. A few years 
later the claims now composing the Jessie group appear to have passed into the hands of the 
Gold Run Mining Co., which early in 1892 was succeeded by the Jessie Gold Mining & Milling 
Co., capitalized at $400,000. A few shipments were made during that year, a new mill was 
finished in 1894, and the mine appears to have been steadily productive during 1895-96. In 
1897 the company suspended operations, and in the following year the property was leased to 
the Dania Gold Mining Co. In 1899 the mine was leased to B. S. Revett, who remodeled the 
40-stamp mill, added concentrating tables to the plates previously in use, and worked the mine 
for a time by stoping only the richer sulphide streaks, together with such oxidized ore as was 
available. In 1906 the Jessie mine was leased to James T. .Hogan, who assigned the lease to 
the Jessie Consolidated Mines Co., capitalized at $1,500,000. Tliis company, however, does 
not appear to have resumed mining and no work whatever was in progress in 1909. The total 
production of the Jessie has been variously estimated at $800,000 to $1,500,000. 

The principal reasons why the mine has not been more steadily and successfully worked 
appear to be the excessive dead work required to find the higher-grade streaks, the irregularity 
and frequent lack of persistency of these when found, and failure to obtain a high percentage 
of extraction in the mill. From all that can be learned the extraction prior to 1899 (when 
concentrating tables were added) was only about 40 per cent. Even after concentration was 
introduced the recovery was certainly not over 80 per cent and probably was under 60 per cent. 

UNDERGROUND W^ORKINGS. 

The underground workings of the Jessie are shown in plan in figure 18. Only^ one level, 
that of the Glenwood and Jessie tunnels, is now accessible. Above tliis are some old upper 
levels and stopes, some of wliich open into pits or ^^glory holes.'' About 180 feet below the 
Glenwood level is the Hattie tunnel, sometimes called the Shale tunnel from the fact that it 

» Moody's first work was on t.Tie Bonania and Seminol«*. claims, which do not appear under those name^ on Plate II (In pocket). Whether they 
were never patented or were recorded under new names, as was done in some other cases, I do not know.— F. L. R. 



U. S. QEOLOaiCAU SURVEY 



PROFESSIONAL PAPEn 7S PLATE X 




JESSIE MINE FROM THE SOUTH. 

slope. J. Disbrow open slope. 4. Glenwood tu 

I tunnel. In Ihe foreground is pari of Ihe Gold Rur 



nel. 5. Jessie tunnol. 




IS. PART OF THE SEMINOLE OPEN STORE FROM THE SOUTH. 

shows Ihe irregular fissunng of the porphyry and the absence of any distinct »i 



STOCKWOBKS AND VEINS OF THE GOLD-SILVER-LEAD SERIES. 



145 



is cliiefly in that rock. 
This tunnel and the 
Quincy tunnel, 30 feet 
lower, were both caved 
near their portals in 1909 
and could not be reached 
through the raises con- 
necting them with the 
Glenwood level. The 
total vertical range of 
the workings from the 
Quincy tunnel to the 
crest of the ridge is 
about 350 feet. 

GEOLOGIC RELATIONS. 

All except a wholly 
inconsiderable part of 
the ore from the Jessie 
mine has come from 
quartz monzonite por- 
phyry. As may be seen 
from Plate I (in pocket) 
this porphyry is part of 
a large and exceedingly 
irregular mass that ex- 
tends from Delaware 
Flats southeastward 
nearly to Famcomb Hill 
and is intrusive into the 
Upper Cretaceous shale. 
Some parts of the mass 
cut across the shale as 
dikes, other parts are 
sills, and still other 
parts are too irregular 
to be classed under 
either form of intrusive 
body. 

As shown by figure 18, 
the accessible workings 
of the Jessie are nearly 
all in the porphyry, 
shale being exposed at 
only three localities. 
One of these is in the 
western part of the mine 
at the end of the Semi- 
nole Crosscut No. 1. At 
tliis place the porphyry 
overUes the shale, the 

90047°— No. 75—11 



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GEOLOGY AND ORE DEPOSITS OF BBEOKENKIDGE DISTRICT, COLORADO, 



contact dipping 20° E. It appears to be a contact by intrusion, although there has been some 
later movement along it. In the northern part of the mine the Revett and Whitehead drifts 
both reach the shale, with which the porphyry here is in close igneous contact, cutting vertically 
across the sedimentary beds. Another contact is exposed southeast of the Jessie timnel. Here 
the contact surface dips to the east and is accompanied by some gouge, due to movement since 
the porphyry was intruded. These exposures are too few to afford much information as to the 
shape of the porphyry mass at the Jessie mine. Undoubtedly much more could be learned 
were the Hattie and Quincy tunnels open. These evidently go through much shale, and 
although in the eastern part of the mine the porphyry may continue indefinitely downward, in 
the western part it lies upon shale, perhaps connected with small dikes in that rock. 

QBE BODIES. 

The ore of the Jessie mine consists mineralogically of pyrite, sphalerite, and galena asso- 
ciated with altered porphyry as gangue. The galena as a rule occurs only in the fissures or in 

the immediate vicinity of fractures. Its presence is invariably a sign 
of good ore. Sphalerite, although present chiefly in veinlets, has a 
little wider range than the galena as a metasomatic constituent of the 
altered porphyry. Pyrite occurs both in the fissures and widely dis- 
seminated throughout the altered porphyry. It is generally of low 
grade, the ore formerly mined ranging in tenor from S3 to $6 a ton. 
The ore from the Revett stope, which is reported to have yielded in 
all about $77,000, is stated, in a report made by Mr. Jeremiah 
Mahoney, to have averaged $4.03 in gold and 48 cents in silver to the 
ton. Some idea of the character of the concentrates produced in 
1897 may be had from the sampler analysis given on page 108, 

There is no vein in the ordinary sense. The entire deposit is 
essentially a stockwork in porphyry, oval in plan, with the small end 
of the oval directed to the northeast. The length of the metallized 
area is about 900 feet and its greatest width about 700 feet. Nearly 
all of the porphyry within these bounds is traversed by small veinlets 
(see PI. XXVI, B, p. 126) carrying pyrite, sphalerite, and less galena 
or contains these minerals disseminated through it, particularly as 
metasomatic replacements, accompanied by sericite, in the large 
orthoclase phenocrysts of the porphyry. Along certain zones, how- 
ever, the fissures are more closely spaced and more regular in trend 
than elsewhere, the proportion of sulphides to waste is greater than 
the average for the stockwork as a whole, and it is to these zones, 
many of them having more or less of a lodelike character, that 
s toping has been confined. The general appearance of such a zone, 
showing the branching and crossing of the sulphide veinlets, is represented in figure 19, which 
is a sketch of tlie back of part of the May B. stope. As a rule tliere are no definite walls to the 
stopes, and groups of stringers that may be nearly parallel and present the appearance of a strong 
stringer lode for 100 feet or more along the strike finally die out or merge with another group of 
different trend. The more pronounced zones of fissuring strike from northeast to east. The 
dip in general is to the northwest and ranges from 30° to vertical. A few zones dip to the south- 
east. Most of the veinlets of sulphides are less than an inch in width and fill cracks of no great 
individual persistency. Even some of the larger stringers, up to 4 inches in width, can be traced 
xmtil they die out completely and the short distance within which some zones, stoped to widths 
up to 15 feet, dwindle to a few insignificant close cracks when followed along the strike is a marked 
characteristic of the deposit. Neitlier before nor after tlie deposition of the ore has there been 
any considerable movement along the fissures by the slipping of one wall past tlie other. Con- 
sequently seams of gouge are exceptional and where present occupy fissures that do not belong 




Figure 19.-^ketch of stringer 
lode in the May B. stope of the 
Jessie mine. 



A 



BTOCKWOBKS AND VEINS OF THE GOLJ)-SIL.VER-LtEAD SERIES. 



147 



to the ore-bearing group. Evidently the ore-bearing fissures were formed by stresses that were 
relieved merely by the opening of many small cracks in the porphyry and that were not 
productive of much displacement. 

No important body of ore has yet been foxmd in the shale. The fissures either end at the 
contact or continue in diminished number and smaller size for a few feet into the shale and then 
gradually die out. The Whitehead stope at its northeast end was carried for a few feet into the 
shale, but the quantity of ore found in that rock was evidently small. 

The depth to which the stockwork extends is not determinable from the workings now open 
to examination, but there is good reason to conclude that a large part of the metallized porphyry 
is underlain by shale on the Hattie and Quincy levels in some such way as is indicated in >^ 
figure 20. Whether any considerable body of porphyry extends much below these levels can not 
be determined without reopening them and prospecting by drilling or sinking winzes. Whether 
the entire mass of porphyry within the oval curve defining the general limit of metallization 
could be profitably worked by open-pit or quarrying methods is a question that could be answered 
only after careful and thorough sampling. Obviously the original average tenor of the whole has 
been much reduced by the selective mining of the past. 



■'} 



NW 




FiQUBE 20.— Diagrammatic seotion through the porphyry of the Jessie mine, showing the general character of the flssurlng. 

HAMILTON MINE. 

The Hamilton mine is at the head of Summit Gulch, a mile south of the Swan and about 2 
miles a Uttle west of north from Lincoln. The mine attracted little notice until about 1893, 
but from that time until the summer of 1899 it appears to have made steady shipments of con- 
centrates from a little 10-stamp mill at the mine. After lying idle for 10 years the Hamilton 
mine has recently been reopened by the White Cloud Mining Co. The total gross production 
is given by the present owners as about $400,000. 

So far as could be learned there is no complete map of the xmdergroimd workings in exist- 
ence. The development is by three main tunnels and one sublet el, all on the east side of the 
gulch. The upper or No. 1 tunnel, from which most of the ore was taken in early days, is about 
200 feet above the bottom of the gulch, where are situated the mill and the portal of tlie lowest 
tunnel, known as the Tip Top tunnel or fifth level. About halfway between these two levels 
is the Blacksmith tunnel, or third level. The present operators are making no effort to 
reclaim the old upper workings, but are confining their attention to exploratory work from the 
Tip Top tunnel and from another tunnel, known as the Homestake, that lies south of the Tip 
Top and 60 feet higher. 



148 



GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 



+ .   + St.ringers 



N 



The No. 1 tunnel crosscuts in a northeast direction a series of nearly east-west veins, which 
in order from south to north are the Sheridan, Surprise, Gulch, and Righthand veins. The 
strikes of these vein range from N. 85° E. to N. 63° E. They dip north at angles ranging from 
75° to vertical. These veins are stringer lodes in quartz monzonite porphyry, which is part of 
the same intrusive body that contains the Jessie and Cashier ore bodies. The lodes have no 
definite walls, are generally less than 50 feet apart, and are merely zones along which the fis- 
suring has been greater than in the intervening slabs of porphyry. In some places two of the 
so-called veins, elsewhere fairly distinct, come together and form a single large ore body. The 
old stopes on the veins, as nearly as could be estimated without maps or measurements, are up 
to 150 feet long, from 5 to 15 feet wide, and extend from points near the surface down to tlie 
third level, or through a vertical range of 150 to 200 feet. In general the stopes on one fissure 
zone are nearly opposite those on another. 

One noteworthy geologic feature of the mine is the occurrence of large blocks of black 
Upper Cretaceous shale in the porphyry. These are bounded in part by igneous contacts and in 
part by faults that have brought the shale into juxtaposition with the porphyry. No ore of 
any importance is known in the shale and those who have worked in the mine stated that the 

veins end at the shale. At one place 
accessible in 1909 the Sheridan vein was 
observed to end at the surface of a body 
of shale, but the contact at this place 
appeared to be due to faulting of later 
age than the deposition of the ore. 

The ore is similar in general charac- 
ter to that of the Jessie and Cashier mines, 
consisting of stringers of pyrite, sphaler- 
ite, and E^alena in pyritized and sericitized 
po;phyr;. There is more pyrite and less 
sphalerite than in the Jessie, and the best 
ore from the old stopes below the No. 1 tun- 
nel shows incipient oxidation. Although 
most of the sulphides fill fissures, they 
have to some extent replaced the por- 
phyry. Some crude ore, shipped in 1903, 
probably partly oxidized, had approxi- 
mately the composition given in the table 
on page 108. 
The concentrates, which have been the chief product from the mine, owed their value mainly 
to their contents in silver and gold, the lead and zinc having been almost negligible. 

The newer exploratory work on the Tip Top level is shown in plan in figure 21, made from a 
rough compass survey. It will be seen that the tunnel passes for about 125 feet through a body 
of shale, which is bounded on the north by a close igneous contact and on the south by a fault 
associated with about 6 inches of soft gouge. In the shale near its north contact with the por- 
phyry is a Uttle tight stringer of pyrite wliich was stated to have afforded 2 ounces of gold and 
8 ounces of silver on assay. The porphyry south of the shale is traversed by small stringers 
of pyrite with a little galena, quartz, and calcite. The largest of these stringers is supposed 
by those in charge of the development to be what is known above as the Sheridan vein. 
This, however, is open to question. 

CASHXEB MINE. 

The Cashier mine is on the east side of BrowTis Gulch, two-thirds of a mile south of the 
deserted settlement of Swan City and 1 mile east of the Hamilton mine. The workings are 
cliiefly on the Casliier and Smuggler claims. Mining on these claims began in the early eighties, 
but the mine attained no prominence imtil about 1898, when a 40-stamp mill was begun. This 




6*porphyry gouge 



^te-Fault 



zoo Fec"t 



Supposed 

^Shecidan 
♦*veirK 



+ + 



FiOTTRE 21.— Geologic sketch plan of the Tip Top tunnel of the Hamilton 

mine. 



STOGKWOEKS AND VEINS OF THE GOLD-SI LVEB- LEAD SERIES. 



was finished late in the following year, and 
although only 20 stamps were actually used, the 
mine was actively worked for about 5 years. It 
was then abandcined, and in 1908 the mill was 
torn down and the machinery removed. The 
total output has not been ascertained but was 
probably between $200,000 and $500,000. 

The mine is developed by two tunnels and 
an intermediate level, the general plan of the 
underground workings as they were in 1902 
being shown in figure 22. In 1909 there was 
no one whatever at the mine, and the lower 
tunnel, about 100 feet above the bottom of the 
gulch, could not be entered. A little sloping 
apparently was doue from thb level, but most 
of the ore extracted came from above the inter- 
mediate level, which is about 60 feet above the 
first or lowest level. Tlie third level k about 
75 feet above the intermediate level and in the 
eastern part of the great stope is from 80 to 100 
feet below the surface of the hill. 

All the Cashier workings that were accessi- 
ble in 1909 are in quartz monzonite porphyry 
forming part of the same irr^ular intrusive 
mass that contains the ore of the Jessie mine. 

The ore body, like that of the Jessie, is 
a stockwork, the ore consisting of fractured 
porphyry traversed by stringers of pyrite, sphal- 
erite, and a little galena. The dominant fissur- 
ing strikes northeast, but owing to the height 
and size of the stopes and the absence of fresh 
working faces little can be ascertained regard- 
ing the details of the fissuring and veining. It 
is certain that the fracturing is irregular and 
that the ore bodies have no definite walls. The 
principal pay shoot was cut in the upper tunnel 
about 150 feet from the portal and was stoped 
for a distance of over 200 feet from southwest 
to northeast, to widths of more than 60 feet. 
The stope is untimbered and forms a vast 
irregular chamber extending from the second 
level nearly to the surface. This impressive 
cavity ia only partly represented in figure 22, 
having been considerably enlarged, especially on 
the second level, since the surveys were made on 
which that map is based. 

The ore of the Cashier mine is of low aver- 
age grade and has a smaller ratio of silver to 
gold than in the Jessie mine. Pyrite ia the prin- 
cipal sulphide and both sphalerite and galena 
are apparently less abundant than in the Jessie 
ore. Occasionally small rich bunches were found 
and some specimens from the upper workings of 
the Cashier, preserved in Breckenridge, contain 
wire gold embedded in galena and sphalerite. 




150 GEOLOGY AND ORE DEPOSITS OF BBECKENBIDGE DISTBICT, COLORADO. 

The approximate composition of a shipment of oxidized ore from the Cashier mine made in 
1904 is given on page 108. 

I. X. L. 



The I. X. L. mine is on the south side of Swan River, a little less than half a mile east 
of the mouth of Browns Gulch and directly northeast of the Cashier, the ends of the Cashier 
and I. X. L. claims being only about 300 feet apart. 

Work on the I. X. L. plaim began in 1881, and the mine was equipped with a mill in 1883. 
Shipments of concentrates continued up to the year 1898, since when the mine has lain idle, 
the upper workings have caved, and the mill has fallen in ruins. The I. X. L. appears 
never to have been one of the larger shippers and, like many other mines in the district, suffered 
from mismanagement. Its total production is not known. 

The principal underground workings consist of two tunnels. The upper one, which enters 
the hiU about 300 feet above the meadows of the Swan and connects with stopes that supplied 
most of the ore formerly milled, is no longer accessible. A newer tunnel, about 200 feet lower, 
runs S. 32° W. for 800 feet and connects through 100 feet of drift to the west with a stope on 
an ore body lying nearly under the ore stoped from the upper level but not known to be part 
of the same pay shoot. 

The geologic relations of the I. X. L. ore are very unsatisfactorily revealed. The lobe of 
shale shown southwest of the portal of the lower I. X. L. tunnel on Plate I (in pocket) is 
probably underlain by porphyry in igneous contact with it, for no shale occurs in the tunnel. 
South of the tunnels the porphyry as exposed in the vicinity of the three shafts shown in line 
on Plate I evidently contains included blocks of quartzite, but the shapes and dimensions of 
these blocks are not shown by surface exposures. The lower tunnel is mainly in quartz 
monzonite porphyry that is part of the same mass within which are the ores of the Cashier, 
Hamilton, and Jessie mines. This porphyry is thoroughly and irregularly fissured and contains 
many small stringers of sidphides. The ore body on this level, however, is a mass of quartzite 
fully 100 feet across from east to west and from north to south. How much larger it is and 
what its thickness is can not be determined without further development. This mass of 
quartzite was intricately fissured at the time of the porphyry intrusion, and the fissures were 
filled with the magma. This complex mass of quartzite, traversed by countless branching and 
crossing dikelets of porphyry, was subsequently shattered, and the resulting fiissures and irregular 
interstices were filled with pyrite, sphalerite, galena, and quartz, with a little chalcopyrite, and 
locally some bismuthinite. Further details regarding this ore have been given on page 108. 

No information could be obtained as to the tenor of the ore, which evidently belongs to 
the low-grade milling class. 

WniE PATCH MINE. 

The Wire Patch mine is situated on the southwest slope of Farncomb Hill, 4 miles east of 
Breckenridge. It takes its name from the Wire Patch placer claim or ''patch," from which 
the main tunnel has been driven into the hill under the Frederick the Great, Elephant, Emperor, 
Queen of the Forest, and other lode claims. The Elephant ore body, from which has come 
most of the production of the Wire Patch mine, appears to have been discovered about the 
year 1882, and by the end of 1886 was credited with an output of $75,000. The total product 
from the Emperor claim at this time amounted to about $15,000 and that from th© Queen of 
the Forest to $8,000. In 1888 the Wire Patch Gold Mining & Milling Co. began operations 
with a new Huntington mill. The mine was idle in 1889, and although operations were resumed 
in 1890 the company apparently was not successful, and in August the mine and mill were 
leased to Henry Farncomb. For the next few years there is little mention of the mine in local 
annals, although it is known to have been worked by lessees in 1897 and to have been reopened 
after a period of idleness in 1899. In 1908 the property was under lease to the Pitt Ores Co., 
of Pittsburg, which carried on some development and milled some ore, but the mine was again 
idle in 1909. 



STOCKWORKS AND VEINS OF THE GOLD-SILVER-LEAD SERIES, 



151 



The general plan of underground development 
of the Wire Patch mine, including also some of 
the old tunnels of' the Ontario mine, is shown in 
figure 23, compiled from several imperfect maps, 
no complete and accurate plat of the workings 
being in existence. The lowest or Mill tunnel 
enters the south slope of Famcomb Hill a little 
over 100 feet above French Creek and runs N. 35° 
E. for 596 feet. This tunnel was projected to 
reach ore bodies known in the upper workings, but 
has never supplied any ore. 

About 126 feet above the Mill tunnel is the 
Elephant tunnel, which at points about 300 and 
500 feet from its portal connects with irregular 
stopes in quartz monzonite porphyry. The Ele- 
phant tunnel pursues a devious northeasterly 
course through the hill and connects at its face 
through a raise with the Grove tunnel, about 200 
feet higher, whose portal is on the north side of 
the hill. The Grove tunnel has caved in. The 
Smith-Emperor and Queen tunnels, on the south- 
west slope of the hill above the Elephant tunnel, 
have long been abandoned. They connect with 
the bottom of an open pit, about 100 feet long 
and 50 feet deep, in an ore body on the Queen of 
the Forest and Emperor claims. Still higher up 
the hill and west of the tunnels just mentioned is 
the long-disused Ontario tunnel, which follows a 
general easterly course and crosscuts six or more 
small veins in shale. 

The rocks of the Wire Patch mine are Upper 
Cretaceous shale intruded by quartz monzonite 
porphyry. The shale has a general dip to the 
northeast of about 25°, It is, however, much dis- 
turbed by the intrusion of the porphyry, which 
cuts it irregularly, as may be seen from the rela- 
tions of the two rocks shown in figure 23. A 
characteristic feature of the porphyry near its con- 
tact with the shale is the inclusion of very abun- 
dant fragments of the latter rock. In some places 
there is a gradation, through a distance of 100 feet 
or more, from porphyry containing a few bits of 
shale to a mass of shattered shale held together by 
a mesh of porphyry and having the general appear- 
ance of a breccia. The porphyry magma, which 
appears to have been erupted in a fairly fluid con- 
dition, evidently split of! and caught up fragments 
of the fissile shale as it invaded that rock, and 
finally solidified as a material consisting in some 
places of more shale than porphyry. This in- 
trusive breccia is well displayed in the Elephant 



o 

M 
S3 






a. 

•d 
p 

o 
>r 
5- 
'5 

o 



cr 

I 

6 

O. 

O 

D 

a 

o 



S 

I 



8 

a 



3* 

I 

O 

3 

ST 

6 

o 



o 
3 




152 GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

tunnel and in the stopes above that level. After passing for 270 feet through shale the tunnel 
enters porphyry crowded with shale fragments. On the tunnel level this material is not much 
metallized, but just above is the Elephant ore body, a large irregular mass that is said to have 
been stoped for over 200 feet eastward and up to the surface. Only a very small part of 
these stopes could be seen in 1909, and whether they connect with the open pit on the hillside 
just above the Queen tunnel could not be ascertained. Much of the ore consists of shale frag- 
ments surrounded by a shell of pyrite, sphalerite, and galena, the sulphides having been depos- 
ited on the surface of the shale by replacement of the porphyry. 

After first entering the porphyry, the Elephant tunnel continues in a general east-northeast 
direction through this rock for about 350 feet and then again reaches shale. For 150 feet from 
this second contact the porphyry is crowded with shale fragments, and partly in this material, 
partly in the porphyry above the tunnel, is another irregular ore body, which has been stoped 
by the present lessees. This stope is under the southwest end of the open cut but, so far as is 
known, does not connect with it. The two principal ore bodies of the Wire Patch mine thus 
occur on opposite sides of an irregular porphyry dike, which, where crossed by the Elephant 
tunnel, is about 350 feet wide. The western or Elephant shoot dips to the east at about 75°, 
and the eastern shoot apparently dips to the west at about 45°. Ore bodies so irregular in 
form, however, can not be expected to maintain a definite dip or pitch for any great distance. 
The hope that the two ore shoots would come together below was influential in determining the 
driving of the Mill tunnel. This tunnel, after going through shale for 516 feet, entered metallized 
porphyry containing shale fragments. Apparently the results were not altogether encouraging. 
The air in the Mill tunnel was too foul in 1909 to permit an examination of this porphyry. 

Beyond the east ore body the Elephant tunnel goes through 100 feet of shale and then again 
into porphyry, within which it follows for 150 feet one of the northeast fissures that were formerly 
productive of rich gold ore on the Ontario claim. It is to be noted that this part of the tunnel 
is all in porphyry, whereas the Ontario tunnel, 300 feet higher, is mostly in shale. Not only 
are the intrusive bodies of porphyry highly irregular in form, but they include, in addition to 
the numerous small fragments of shale already referred to, some large blocks of shale that are 
possibly 100 feet or more across, although none is well enough exposed on all sides for satisfactory 
measurement. 

The only place in the Wire Patch mine where the occurrence of the ore could be at all satis- 
factorily examined in 1909 was in the east stope, about 25 feet above the Elephant tunnel. 
Here a series of irregular, untimbered connected chambers had been worked out in the porphyry, 
which at this particular place is not so crowded with shale fragments as in the part of the Ele- 
phant stope seen. The ore consists of sericitized porphyry carrying pyrite, sphalerite, galena, 
and occasionally a little pale-pink carbonate of manganese and iron that may be designated 
impure rhodochrosite. These minerals in part fill numerous irregular fissures and interstices 
in the fractured porphyry, but to a considerable extent they have metasomatically replaced that 
rock, the process working outward from the many fractures. Pyrite is by far the most abun- 
dant sulphide, and galena is rather rare, occurring here and there in bunches composed of crystals 
up to an inch across. Some of the pyrite is as coarsely crystalline as the galena. 

The ore is concentrated by jigs and tables to a product that in 1908 was running about 
0,9 ounce of gold and 10 ounces of silver to the ton, with 34 per cent of iron and 10 per cent 
of silica. 



CHAPTER XI. 
THE FARNCOMB HILL GOLD VEINS. 

DISTRIBUTION AND GEOLOGIC ENVIRONMENT. 

Deposits of the Faxncomb Hill type are not known outside of the area that embraces the 
northeast slopes of Famcomb and Humbug hills. The really important veins have an even 
more restricted range, being limited to the western part of Famcomb Hill and to an area 2,500 
feet long and less than 1,500 feet wide, the southern side of which may be considered as lying 
generally along the crest of the hill from the Ontario saddle on the west-northwest to the head 
of Dry Gulch on the south-southeast. If the productive parts of the veins only are taken into 
accoimt a rectangle 2,500 feet long and about 500 feet wide might be laid out across the fissures 
so as to include all the pay shoots. The general trend of the veins is nearly northeast. The 
principal ones recognized from west to east are the Ontario, Key West, Boss No. 2, Boss, 
McQuery, Reveille, Carpenter, Gold Flake, Graton, Silver (or West Bondholder), Bondholder, 
and Foimtain veins. These fall into two groups, of which the western one, comprising the 
Ontario, Key West, Boss No. 2, Boss, McQuery, and Reveille veins, is separated by an interval 
of 700 to 800 feet from the eastern group. 

The geologic relations of the veins may best be understood by reference to the general 
geologic map (PI. I, in pocket). It will there be seen that a considerable portion of the western 
part of Famcomb Hill is composed of quartz monzonite porphyry, all of which is more or less 
altered and weathers with characteristic rough, pitted surfaces. This porphyry is an irregularly 
shaped mass which connects with a larger body along the crest of Humbug Hill and with some 
small sheets and dikes. The Famcomb HiU mass is intrusive into dark Upper Cretaceous shale 
that strikes generally from N. 10° W. to N. 30° W. and dips northeast at an average angle of 
30°. The porphyry body, as a whole, shows some tendency to conform with the general dip of 
the shale, but in most places it clearly cuts across the beds and probably somewhere continues 
almost vertically downward to abyssal depth. In other words, it appears to occupy the site 
of one of the minor conduits that supplied magma for the porphyry intrusions. On the north, 
the map shows two sharp projections from the mass. The longer and more slender one, extending 
well down the slope toward Georgia Gulch, is a dike. The shorter one is probably merely the 
upper sloping surface of the main mass, exposed by erosion at the head of American Gulch. 

All aroimd the porphyry the shale is thoroughly brecciated and nearly everywhere there 
intervenes between the solid porphyry and the undisturbed shale a zone of passage from por- 
phyry containing a few fragments of shale to shattered shale cemented by porphyry. This is 
clearly an intrusional phenomenon and is not a result of brecciation after the solidification of 
the porphyry. The igneous material caught up the shale fragments or penetrated the inter- 
stices between them, and, while still in a molten condition, bound the shattered material 
together. Nevertheless, the sedimentary rock shows no perceptible crystalline metamorphism, 
partly because it is not especially calcareous but chiefly, doubtless, because the porphyry magma 
was intruded at low temperature and solidified with comparative rapidity. This suggests that, 
although there may have been a direct connection with magmatic sources by a conduit under 
Famcomb Hill, this conduit could scarcely have been of any great size and could not have 
afforded egress to large quantities of molten material. The zone of shale fragments is in some 
places fully 100 feet broad and generally shows some metallization. Prospect pits in it as a 
rule reveal a Uttle chalcopyrite partly changed to malachite, and the Elephant ore body of the 
Wire Patch mine is essentially a metallized part of this breccia zone. The shale fragments in 

153 



154 



GEOLOGY AND ORE DEPOSITS OF BEECKENRIDGE DISTRICT, COLORADO. 



the porphyry are in part sharply angular, in part rounded and pebble-hke. The rounded ones 
commonly exhibit a faint superficial concentric banding, showing that some Uttle chemical 
change has taken place in them close to the porphyry matrix. As a rule the fragments have 
lost their original nearly black color and are light gray. 

The rich gold veins Ue near the porphyry mass on its north side. At the head of G^rgia 
Gulch, in an embayment of shale between the Famcomb and Humbug Hill masses of the 
porphyry, are the veins of the Ontario, Key West, and Boss group. At the head of American 
Gulch, lying between the Farncomb Hill porphyry mass on the west and a long, slender dike of 
monzonite porphyry on the east (see PI. I, in pocket), are the Gold Flake, Graton, Silver, 
Bondholder, Fountain, and other veins of the eastern or Wapiti group. It is doubtful whether 
any important pocket of gold has been found more than 300 feet from the main body of por- 
phyry. On the other hand, although some of the veins imquestionably enter the porphyry, 
they have never, so far as could be ascertained, proved productive in that mass, although gold 
has been found in the veins where they traverse some of the relatively thin sheets of porphyry 
intrusive in the shales. Between the two principal groups of veins there is at least one other 
vein known, the Kingfisher, which traverses the little tongue of shale that, as shown on Plate I, 
projects into the porphyry just west of the head of American Gulch. This vein dips northwest, 
like those of the western group, and shows a few copper stains, but it is not productive. 

UNDERGROUND WORKINGS. 

Although some shallow shafts, including the Ontario, were sunk on the veins in the' early 
stages of development, it was soon found that adits were more economical and convenient. 
There are now a large number of adits on the north side of Famcomb Hill (see PI. XVII, B, 

p. 76) , but the workings gen- 

)f 



/ 



Dip of shale 
It 



erally are in poor condition 
and can only in part be ex- 
amined. Even where the 
portals of the tunnels have 
not caved or been covered 
by later dumps they are 
sometimes completely closed 
by accumulations of ice. It 
is long, moreover, since any 
acciu'ate surveying was done 
in the hill and in the mean- 
time lessees have burrowed 
here and there until there 
is little correspondence be- 
tween present conditions and 
the latest available plats. 

A part of the old Ontario 
workings is shown in fig- 
ure 23 (p. 151), in connec- 
tion with those of the Wire 
Patch mine. The Ontario 
tunnel, although partly filled 
with ice, could be examined 
in 1909. It appeared that 
stoping had been done on 
five or six small short veins 
in a body of shale which is bounded on the east and west by porphyry. The Grove tunnel, on 
the north side of the hill, about 80 feet below the Ontario tunnel and in the neighborhood of 
100 feet below the saddle between Famcomb and Humbug hills, is completely closed. 





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FiaxTBX 34.— Sketch plan of the lower Key West tunnel, showing character of Assuring. 



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THE FABNCOMB HILL GOLD VEINS. 155 

The Key West vein has been worked through three or four small tunnels of which the 
lowest, shown in plan in figure 24, is a good example. The upper workings have largely fallen in. 

The Boss workings are similar in character and condition to those of the Key West. The 
upper Boss tunnel enters the hill from the road (see PI. I, in pocket) and, although it was not 
entirely closed in 1909, very Uttle information was obtainable in it. Below the road are two 
or three other tunnels which formerly gave access to the Boss vein, but all are now blocked. 
Eecently some lessees have partly opened one of these so as to reach and work the Reveille 
vein, which lies southeast of the Boss vein. 

The workings of the Wapiti group are all on the American placer claim (also known as 
Fuller & Greenleaf placer No. 85), which was patented before the existence of the veins was 
known. They are shown in part in Plate XXX, based on old surveys and far from complete. 
The workings have a vertical range of about 434 feet from the collar of the old Bondholder 
shaft to the Fair tunnel. Scarcely any of the tunnels above the Fair can now be entered 
through their portals. The Fair tunnel, after crosscutting for 750 feet through shale with a 
few limestone beds and some regular sills of porphyry, reaches the Gold Flake vein; thence it 
runs nearly south to the Silver vein. On both veins there are raises that connect with an 
intermediate level 90 feet above the Fair tunnel. On this level is a 200-foot southeast crosscut 
that passes entirely under the old Bondholder workings. From the Intermediate level access 
may be had through raises to the Wheeler and Simondson tunnels. 

COUNTRY ROCK. 

The general country rock of the Farncomb Hill veins is a variety of the Upper Cretaceous 
shale which is not by any means confined to this hill but may be matched by material from 
Kocky Point, Sununit Gulch, and many other widely scattered locaUties in the district. It is 
so dark a gray as ordinarily to be termed black. It is for the most part sHghtly calcareous but 
locally grades into material that might be called impure shaly limestone. Where exposed for 
a short time to the weather, as in the old placer workings of Georgia Gulch, the shale flakes 
and crumbles, but in underground exposures, out of reach of the weather, it is a fairly hard and 
firm rock, as may be seen in the Fair tunnel. Miners familiar with Farncomb HiU recognize 
certain peculiarities in that shale which they regard as possibly gold bearing. In mass it must 
have a brownish hue rather than the dull grayish black of the normal fresh shale. This tint, 
as close examination shows, is due to films of iron oxide on fracture planes. The rock favor- 
able for the occurrence of a pocket also breaks readily into small angular fragments and gen- 
erally presents the appearance that a geologist recognizes as marking an incipient stage of 
oxidation or weathering. Although the miner may regard it as a distinct variety of shale, the 
ore-bearing rock is not essentially or primarily different from most of the shale in its vicinity. 

Within the shale are sills or sheets of quartz monzonite porphyry, generally from 2 to 20 
feet thick, which are probably offshoots from the main porphyry mass. As a rule these are 
Tegular and dip with the bedding of the shale, as may be well seen in the long crosscut of the 
Fair tunnel. Of less common occurrence are small irregular intrusions of monzonite porphyry, 
one of which is exposed near the northwest part of the Silver vein in the Wheeler tunnel. 
Both kinds of porphyry are more or less decomposed and neither appears to have metamor- 
phosed the shale at its contacts. 

THE VEINS* 

The Farncomb Hill veins are remarkable for their small size, being rarely over half an inch 
wide. Nevertheless, they cut directly across the bedding of the shale and through the porphyry 
sills without being deflected into the planes of bedding or contact. For such narrow fissures also 
they are surprisingly regular and persistent. The Gold Flake, one of the strongest of the veins, 
ias been stoped or followed for a length of 300 feet and to a depth of about 450 feet. The Silver 
vein is known to be almost as persistent as the Gold Flake. At their ends the veins narrow to 
invisible cracks, some of which are associated with other small, parallel, overlapping veinlets. 
As a rule also the principal veins are accompanied by smaller, approximately parallel fissures 
on one or both sides or send out spur veins at small angles. This characteristic is well illustrated 



156 



GEOLOGY AND OBE DEPOSITS OF BRECKENBIDGE DISTRICT, COLORADO. 



^'jSh.b 



by the K^ West vein, as shown in figure 24 (p. 154), The Wheeler vein, shown in Plate XXX 
Cp. 154), is a branch from the Gold Flake and the Eccles vein appears to be a similar spur from 
the Graton vein. The Black vein, which carries gold but not in minable quantities, parallels 
the Gold Flake vein on its soutliwest side at a distance of about 20 feet, and nearly every one of 
^^ y .^ the producing veins is associated with sim- 

ilar but less conspicuous veinlets. Here 
and there the veins are compound, there 
being two or more parallel veinlets in a 
total width of I or 2 inches. Moreover, 
where a vein passes from shale into por- 
phyry it may split up into a miniature 
stringer lode up to 4 inches wide, as U the 
case with the most northwesterly vein of 
the Ontario group, shown in figure 23 
(p. 151), and as is illustrated also in the 
generalized section of figure 25. 

The vein fissures were opened with 
no perceptible faulting of the structures 
that they traverse. The Gold Flake vein, 
for example, one of the most persistent in 
the hill, is well exposed in the Fair tunnel 
at a place where it passes downward from 
shale into a porphyry sill. Close exami- 
nation at tliis point failed to discover any 
faulting of the sharp contact between 
shale and porphyry. Perhaps could all 
the veins be tlioroughly examined it might 
be found that some occupy fault fissures, 
but in any case the displacement is prob- 
ably very slight — a distance of inches 
rattier than feet. 

It is as unlikely that fissures so nar- 
row and opened to the accompaniment of 
so little faulting should continue down- 
ward indefinitely as it is tliat they should 
continue for miles along the strike. Their 
character would suggest that as distinct 
fissures they die out at moderate depth, 
and "tlie present workings afford evidence 
that this indeed is probably the fact. In 
the Fair tunnel, between 400 and 500 feet 
below the highest workings on the hill, 
only two of the numerous veins, the Gold 
Flake and the Silver, have been recog- 
nized, although probably some of the 
others might be found with careful search. 
On the Intermediate level, 90 feet above 
the Fair tunnel, the Graton vein has not 
been identified and a long crosscut run 
under the old Bondholder workings failed to cut any recognizable representative of the 
Bondholder veins. 

Tlie veins themselves are faulted by numerous slips parallel with the bedding of the shale, 
or with the contacts of the porphyry sills. Generally these slips are narrow and the displacement 




2 Feat 



FiQUBE 25.— OeoeiBlIiad gactfoQ ofB Famcoinb mil gotd vein. The plans ol the 
sectlOD is perpeadlculai to that ol tlie vein, rspresantsd as dipping 72* SE. The 
sbale strllcra N. IS" W. and dips IS' E. The apparent dip In (he section Is 
accordingly leas than the true dip. TtievsialsshonniisspllCtlOElnloslrinfieR 
Id passing through a small porptariy sill. It is sllehtl; displaced by betiding 
slips accompanied by seams of gouge. The oocurrcoce or the gold pockela 
( black) Is related to these slips and to the porphyry. 



THE FABNCOMB HILL GOLD VEINS. 157 

eflFected by them is small. From all that could be learned from miners familiar with the old 
workings it rarely exceeds 10 feet, and the greatest sUp known is about 35 feet. The hanging 
wall in general appears to have moved down relatively to the footwall, but too little could be 
seen of the workings in 1909 to establish tliis as an invariable rule. The effect of such faulting 
is to give the vein as a whole a steeper attitude than is indicated by its actual angle of dip at 
any one place^ There are a few small faults, also apparently younger than the veins, that cut 
across the bedding of the shale. A so-called porphyry dike, seen imperfectly in the upper Key 
West tunnel in 1909, is apparently one of these fault fissures filled with brecciated and recemented 
shale. 

The vein material is generally wholly or partly oxidized and comparatively little can be 
learned directly of the character of the veins before oxidation. The gangue material, as shown 
by parts of veins that have escaped oxidation and by comparatively barren stringers accompany- 
ing the principal veins, was wholly or chiefly calcite. Sulphides known to have been present 
in the veins are pyrite, chalcopyrite, sphalerite, and galena. Parts of the Silver vein on the 
level 90 feet above the Fair tunnel consist of white calcite carrying pyrite, sphalerite, and 
galena. Other parts are solid, nearly black sphalerite with a Uttle galena and less pyrite. The 
Reveille and some of the Ontario veins appear in places to have consisted cliiefly of chalcopyrite. 
The oxidized filling of many of the veins shows copper stains and some of it contains a Uttle 
unoxidized chalcopyrite. Crystalhne native gold has been foxmd from time to time embedded 
in calcite, sphalerite, or galena. Thus the veins, prior to oxidation, appear to have contained 
the four sulphides mentioned, in various proportions, accompanied by native gold in a calcite 
gangue. In the oxidized parts of the veins the calcite and sphalerite have as a rule disappeared, 
though some of the pyrite, chalcopyrite, and galena may remain. Generally the vein material 
is more or less spongy or earthy limonite in which occur the masses of native gold described on 
pages 81-82. Chemical tests on oxidized material from the Silver and Reveille veins show that 
there is little or no manganese present. 

The pockets of native gold for which the veins are exploited are very clearly related to the 
small faults that dislocate the veins and to the porphyry sills. Prospectors seek and explore 
the intersections of veins with the bedding sUps and with porphyry sills and expect to find the 
veins practically barren away from these structural features. The workings seen in 1909 indi- 
cated that the pockets occur generally above the slips, which, in some places, divide oxidized 
material above from a calcite vein below. The testimony of those who have worked in the hill, 
however, is that pockets may he below as well as above the slips and sills. Gold, moreover, 
may occur in that part of a vein which crosses a porphyry sill. 

The gold is remarkably segregated in these veins. Most of the vein material contains too 
little of the metal to pay for stoping, but here and there are the famous pockets where a section 
of the vein 2 or 3 feet in diameter and up to an inch tliick may consist of a nearly continuous 
hackly mass of crystalline gold ramifying through a matrix of limonite. When such a pocket is 
found the toil of months may be richly repaid by gold to the value of several thousand dollars 
removable in a few hours and convertible by the miner himself directly into bulhon. Although 
it is now many years since any very rich pocket was found, lessees working the Reveille vein 
recently took out about $1,000 worth of gold from a section of the vein about 3 feet long and 2 
feet broad. Tliis pocket was found close to a little decomposed porphyry dike that cuts the 
shale. Another pocket, worth from $3,000 to $4,000, was found a few months earlier by the 
same lessees on the Boss ground. 



CHAPTER XII. 

VEINS IN THE PRE-CAMBRIAN ROCKS AND OTHER DEPOSITS NOT DESCRIBED 

IN THE PRECEDING CHAPTERS. 

VEINS IN THE PRE-CAMBRIAN ROCKS. 

QENEBAL FEATX7BES. 

The deposits in the pre-Cambrian" crystaUine rocks are generally rather narrow fissure 
veins carrying auriferous pyrite and in some places free gold in a quartzose gangue. Other 
sulphides occurring with the pyrite are sphalerite, galena, chalcopyrite, and bismuthinite. 
Not all of these, however, were noted in any one deposit. Veins of this group have not proved 
of great importance in the vicinity of Breckenridge, and only one, the Laurium, was being 
worked within the mapped area in 1909. 

BXAMPLBS AND MINES. 
LAUKIUM MINE. 

As noted in Chapter I (p. 17^, the Laurium mine, in lUinois Gulch, was one of the first to 
be developed in the district and its original owners shipped some lead ore obtained near the 
surface. After many years of idleness it has recently been reopened by the Blue Flag Mining 
Co., which in 1909 was concentrating the ore in a 60- ton mill and shipping the concentrates, 
stated to have a gross value of $32 a ton. The total product of the mine to the end of 1909 is 
given as about $80,000. 

The workings are practically all on one adit level, shown in figure 26. The first vein cut 
in the adit is called the Porphyry vein; it strikes nearly east and west and dips 75° N. This 
is a zone, up to 1 foot wide, of small fissures, some of which carry galena, sphalerite, and 
pyrite. The vein cuts across the foliation of the schists, which here strikes north and dips 
75°-80° E. A very little stoping has been done on the vein, which is cut off to the east by 
a strong faidt fissure, as shown in figure 26. This fissure, known as the Lead King vein, 
although clearly younger than the fissure of the Porphyry vein, carries in its soft gouge and 
crushed rock considerable disseminated pyrite and some streaks and bunches of sphalerite. 
This is not merely dragged ore but has apparently been deposited in the fault gouge, although 
it shows some later crushing by movement along the fissure. This soft vein has been stoped, 
but the old workings have been closed. 

From the Porphyry vein a crosscut through granite and gneiss gives access to the Laurium 
vein, which in reality is not a single lode but a chain of small veins that pinch out and overlap 
with considerable variation in strike and dip. Along most of its course on this level the vein 
is narrow, and in some places it is filled with little more than a fiilm of gouge. At one place, 
shown in figure 26, it is apparently offset about 75 feet by the Lead King vein or fault. It 
is not certain, however, that the fissure followed east of the fault is the same as that previously 
drifted on; the two dip in opposite directions. East of the offset the Laurium vein for 500 
feet, as exposed in the drift, contaios very little ore. At two places, as shown in figure 26, 
the vein has been followed to a pinch and then another fissure has been picked up by cross- 
cutting. The main drift continues east for over 100 feet beyond the point shown in figure 26 
and here some stoping was in progress in 1909, the stopes at that time being from 2 to 4 feet 

158 



VEINS IN THE PRE-CAMBRIAN EOCKS. 



159 



wide and up to 40 feet high. The ore at this place 
consists of a much-altered rock, possibly a fine- 
grained dike, containing disseminated pyrite and 
traversed by many irregular little stringers of pyrite, 
sphalerite, galena, and quartz. Under the microscope 
the rock is seen to be essentially a mixture of quartz 
and a ferruginous carbonate. This rock contains Uttle 
indefinite green spots, possibly due to the presence of 
fuchsite, or chromium mica, a mineral noted by Spurr 
and Garrey ^ in the wall rock of some of the lodes of 
the Georgetown quadrangle. The material in the 
Laurium mine, however, is too obscure for satisfactory 
determination and may be merely a greenish sericite. 

SENATOR MINE. 

• 

The Senator mine is 8 or 9 miles south of Breck- 
enridge, on North Star Moimtain. A little stoping 
has been done here in the past and the mine is equipped 
with a small mill, which has been idle for some years. 
A general plan of the workings is shown in figure 27. 
The adits generally follow the vein. The general coun- 
try rock is fine-grained biotitic gneiss, very irregularly 
cut by various pegmatites. 

The Senator vein strikes N. 6° E. and dips 67 "^ E. 
Its width varies greatly and attains a maximum of 
about 4 feet. There is practically no gouge on either 
wall. The Senator vein is crossed without apparent 
displacement by the Witch Hazel vein, which strikes 
nearly east and west and is nearly vertical. This vein 
resembles the Senator but has a width of 16 feet in 
one place. Another cross vein, very slightly explored, 
lies about 270 feet south of the Witch Hazel, as shown 
in figure 27. 

The uppermost tunnel is perched high on the steep 
slope of the mountain overlooking the Blue, with its 
portal just below the base of the Cambrian ''Sawatch" 
quartzite, which here rests directly on the crystalline 
rocks with no intervening conglomerate. These upper 
workings show that the vein extends into the quartzite, 
but it is narrower in that rock than below and at the 
face of the tunnel virtually dies out. Considerable 
oxidized ore is said to have been taken from the vein 
in the pre-Cambrian, just under the quartzite. 

The veins as exposed in the tunnels below No. 1 
are hard, tight, and unoxidized. Their principal con- 
stituents are quartz and pyrite, the latter in some 
places forming nearly the whole of the vein. Asso- 
ciated with these minerals are very subordinate quan- 
tities of chalcopyrite, sphalerite, galena, and specular- 
ite, with probably some magnetite. According to 
M. M. Howe, who was doing some work in the mine in 
1908, the ore averages from 1.5 to 2 ounces of gold to 



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1 Prof. Paper U. 8. Qeol. Surrey No. 63, 1906^ p. 143. 



160 



GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 




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the ton, with a small propor- 
tion of silver. This probably 
refers only to the best shoots, 
such as were stoped. 

ARCTIC MINE. 

About half a mile west of 
the Senator mine, in the cliffs 
south of the lower of the two 
lakes in wliich the Blue heads, 
are the workings of the Arctic 
mine, credited with a total 
production of about $50,000, 
although tliis is probably ex- 
aggerated. The Arctic vein 
cuts across the foUation of the 
gneiss, striking N. 15° E. and 
dipping 75° to 80° E. The 
vein, however, is very wavy 
and in places dips west. It 
consists of solid quartz, up to 1 
foot wide, and contains pyrite, 
chalcopyrite,bismuthiiiite, and 
free gold, the last being some- 
times found in handsome speci- 
mens. There is no gouge and 
no evidence of movement since 
the vein was formed. A sec- 
ond vein, known as the West- 
em Star, cuts across the Arctic 
with a more easterly strike and 
lower dip. Neither vein ap- 
pears to fault the other, but 
more underground work will 
be necessary to determine the 
relation of the two veins to 
eaoh other. 

There are four or five tun- 
nels on the Arctic vein, but 
all except the lowest are little 
more than cuts in which a face 
across the vein is exposed to 
dayUght. The lowest tunnel, 
wliich is several hundred feet 
above tlie lake, was being 
driven in 1909 to explore the 
Arctic and Western Star veins 
but had not opened up ore 
when visited. A 10-stamp mill 
had then just been completed 
and this has since been con- 
nected with tlie workings by 
an aerial tramway. 




A. NATURALLY ETCHED SURFACE OF DAKOTA QUARTZITE. 

Natural size. Sbb page 37. 




B. ORE FROM THE SENATOR MINE, NEAR THE HEAD OF BLUE RIVER. 

Nicols crossed. X 30. Coristituents are quaiti (7), calcite (c). chlorite (M), specularite 
and magnetite {mil), and pyrita (p). Drawn with camera lucida, SemidiaErammatic 
See page 159. 



GOLD-SILVER DEPOSITS IN THE DAKOTA QUABTZITB. 161 

LINO MINE. 

The Ling mine is a shoirt distance west of the Arctic and is similarly situated. It has 
been worked intermittently by lessees for many years in a small way and has produced a little 
good gold ore. No examination of it was made. 

BIiANKET DEPOSITS OF GIBSON AND SHOCK HILLS. 

The deposits next to be noticed are of a kind to which the miners generally throughout 
Colorado refer as ** contacts.*' The term as applied to ore bodies was first used at Leadville, 
where the irregular sheets of ore were found to lie near the contact between limestone and 
porphyry and where accordingly there was more reason behind its original application than 
can be found for its present indiscriminate use for any flat-lying deposit in bedded rocks. 

Deposits formed by metasomatic replacement of certain beds in the *' Wyoming" and 
Dakota formations have in the past contributed to the yield of the district. On Gibson Hill 
the Kellogg and Sultana were the chief producers, and on Shock Hill the Iron Mask was the 
leading mine. On Gibson Hill, in the Dakota area, there appear to be at least two horizons 
at which ore deposition has taken place. Nothing whatever can now be seen of these deposits. 
They yielded oxidized or partly oxidized silver-lead ores of the general character illustrated 
by the analysis of ore from the Kellogg mine given in the table on page 108. 

At the west base of Gibson Hill, close to the Blue, are the tunnels of the Sultana and Fox 
Hills mines, in the red micaceous grits and shales of the '* Wyoming" formation, which dip 
gently east, at angles up to 20°, and are stepped down to the west by small normal faults. There 
are two sheets of ore about 5 feet apart and up to a foot or two in thickness. The ore is gener- 
ally pyritic, but the Sultana is known to have shipped considerable partly oxidized lead ore. 
Some of the pyritic ore apparently was also stoped, but it must have been of very low grade. 
The workings of these mines are only a few feet below the bottoms of the old bench-gravel 
placers and are in bad condition, being for the most part closed. There was no opportunity to 
study the details of the replacement of the beds by ore. 

In Shock Hill the Iron Mask ore is said to have occurred above and below a sheet of por- 
phyry. Its general composition is given on page 108. In some places the carbonate ore was 
accompanied by much free sulphur, as described on page 81. Nothing whatever could be seen 
of the ore bodies in 1909. 

The dump of the Finding shaft, on the top of Shock Hill, shows some massive sphalerite 
and abundant massive pyrite, which presumably came from some blanket deposit in the Dakota • 
or *' Wyoming" beds. Evidently the material was not worth stoping. 

GOIiD-SIIiVER DEPOSITS IN THE DAKOTA QUARTZITE. 

At a few places, especially on Shock Hill and Little Mountain, oxidized gold-silver ores 
have been found at slight depth in the fissured quartzite. No large bodies of ore or persistent 
veins are known in this group of deposits. The gold and silver are either distributed rather 
generally along the many small irregular fractures in the little quartzite or segregated at a few 
favorable spots into pockets of exceptionally rich ore, such as was found in the Germania workings 
on Little Mountain. 

In Little Mountain the quartzite, with intercalated beds of gray shale, dips generally a 
little west of south at angles of from 30° to 50° and is cut by fissures trending from north to north- 
east. Some ore has been found in these fissures, but more has come from seams parallel or . 
nearly so to the bedding of the quartzite or from small bunchy pockets. 

The most important workings in Little Mountain are those connected with the Germania 
tunnel (fig. 28), but these are in bad condition and only in part accessible. The tunnel enters 
the west base of the hill close to the river and runs northeast for about 300 feet through bowlder 
till and then through about 140 feet of much-disturbed black shale, quartzite, and monzonite por- 
phyry. The structure of these rocks can not be determined from the present exposures in the 

90047**— No. 76—11 11 



162 



GEOLOGY AND ORE DEPOSITS OP BBECKENBIDGE DISTRICT, COLORADO. 



tunnel. From the main tunnel (fig. 28) a drift turns north on a strong zone of fissuring which 
dips about 55° W. The rocks along this fissure are crushed and exposures in the drift give no 
hint of their general structure. At the winze shown in figure 28 a seam of ore was found in 
hard massive quartzite which was stoped up an incline, as shown. This ore is said to have been 
of good grade, but the body evidently was small. Its general dip was to the south at about 



SOUTH 



E T I G R A D E 




HOUSTON 



Moraine ' ®" "^«>cfi~®5 



aooleet 

-J 

Figure 28.— Plan of the Germania and South Elkhom tunnels In Little Mountain. 

28®, whereas the dip of the quartzite is about 40°. The quartzite has been irregularly frac- 
tured along a plane lying at a little lower angle than the bedding, and the small fissures and 
interstitial spaces, some of them enlarged by solution, have been coated with quartz druses and 
are filled with a porous rusty material that is chiefly limonite. This material, which in places 
carries enough gold and silver to constitute rich ore, is probably an oxidation product of pyrite. 



QOLD-SILVEB DEPOSITS IN THE DAKOTA QUABTZITE. 



163 



The richest bunch of ore ever found in the quartzite of Little Mountain lay within 10 feet 
of the surface in the footwall of the atrong fissure which is cut in the Germania tunnel and on 
whose hanging-wall side was the pay shoot that has just been described. A little flat seam of 
limonite that was followed into the footwall of this fissure was found to expand into an ore 
body which, as shown in figure 29, turned down to the east with the bedding of the quartzite. 
This mass was about 20 feet in diameter and up to 6 feet thick. Around its edge the ore graded 
into spongy limonite of no value. The ore itself differed little from this material in appearance, 
being a very porous mixture of limonite and silica which earned gold and much silver. The 
silver was for the most part probably native or in the form of cerai^yritc, but Thomas West, 
who discovered the pocket, reports that there was also some "sulphurets" of silver — possibly 
ai^entite. A few feet southeast of the ore body the quartzite is cut by monzonite porphyry; 
the ore thus accumulated in an angle between the fissure and the porphyry. 

The South Elkhom tunnel, shown in figure 28, northwest of the Germania timnel, cuts 
porphyry, shale, and quartzite, the distribution of these rocks being indicated in the illustra- 
tion. There is a drift on a fault fissure which strikes northeast and dips 70° NW. The throw 
of the fault has not been de- 
termined, the rock on each 
side being generally the same — 
quartzite and some shale. The 
fissure apparently contains no 
ore. A little north of this tun- 
nel and higher up the hill, prob- 
ably on the South Etigrade 
claim, is another tunnel on a 
similar fissure also dipping 70° 
NW. Oxidized ore to the value 
of $10,000 is said to have been 
stoped from this fissure between 
the tunnel and the surface. 
The country rock b quartzite. 

The composition of some of the ore from Little Mountain may be seen from the table on 
page 108. 

The gold-silver ore in the quartzite of Shock Hill appears to have been generally similar 
in occurrence to that in Little Mountain — superficial, pockety accumulations of oxidized ore 
in the intricately fractured brittle rock. Nothing, however, could be seen in 1909 of the old 
Brooks-Snider and other workings in the hill. The mill appears to have been supplied by 
burrowing unsystematically after small streaks and bunches of ore. A part of the quartzite 
debris was washed by hydraulic means. 

On the north side of Illinois Gulch, just east of the Washington mine, the Dakota quartz- 
ite, here much shattered, has also been washed by monitors or giants for the gold contained in 
its crevices. 

All of the ore found in the quartzite in deposits of this type is apparently the result of con- 
centration and enrichment by the action of oxidizing waters from the surface upon bunches 
and small stringers of auriferous and ai^entiferous pyrite. Some of this pyrite evidently con- 
tained a little copper, probably as chalcopyrite, and the abundant silver in such bodies of ore 
as those found in Little Moimtain suggests the former presence of some galena also. 




FtaUBE 2D.— Sketch cross secllon stnwliig mode ol i 



n the qiurttite ot Little Ifouotaln. 



rich pocket ol oildli«d gold- 



CHAPTER XIIL 

SOME ADDITIONAL FEATURES OF THE DEPOSITS AS A WHOLE AND THEIR 

BEARING ON ORE GENESIS. 

PARAOENESIS OF THE ORE-FORMING MINERALS. 

The ores of the Breckenridge district, owing to their lack of banding, show only obscure 
traces of sequence in the deposition of their constituent minerals. In general, the sulphides 
are irregularly aggregated in a way that at first sight suggests the essentially contemporaneous 
deposition of all. There is neither uniform occurrence of one sulphide as a crust on others nor 
any traversing of older constituents by veinlets of younger ones. Nevertheless, although such 
indubitable evidence of successive depositions is lacking, close inspection of any representative 
face of ore shows that the sulphide minerals are not all strictly contemporaneous. In ore such 
as that of the Wellington mine the formation of jgalena, to a large extent at least, followed the 
deposition of pyrite and sphalerite. In a general way this is indicated by a tendency on the 
part of the galena to occur in bunches having a central cavity or vug lined with the free faces 
of lai^e crystals. • There is a tendency also for the galena to segregate in the middle portion of a 
vein rather than along its walls. The fact that much of the galena is younger than much of the 
pyrite and sphalerite is determinable in a more particular way from small specimens of the ore. 
One of these, for example, in the cabinet of Mr. B. S. Revett, shows a coarse aggregate of pyrite 
and dark sphalerite upon which have grownlarge cubes of galena whose faces are dusted with a 
second generation of small sphalerite crystals. Many other specimens collected in the mine in 
1909 show the same relation between the galena and the older pyrite and sphalerite, although 
the younger sphalerite crystals are not common. No evidence that the development of pyrite 
and sphalerite on an important scale followed the deposition of galena was obtained from the 
Wellington mine, although, on the other hand, many specimens are inconclusive on this point 
or show an aggregation of the three minerals, such as may have resulted from contemporaneous 
growth. In the Wellington veins the siderite (or a related ferruginous carbonate) is younger 
than the important sulphides, as shown by its incrusting them or running through them as 
veins. In some places the carbonate contains a little pyrite in small crystals. A little barite 
noted in the Wellington ore apparently belongs to the very latest period of deposition. 

In the Sallie Barber vein the deposition of the principal sulphide constituents, sphalerite and 
pyrite, was followed by a second and commercially unimportant generation of lighter-colored 
sphalerite and of pyrite. The growth of these was succeeded and partly accompanied by the 
deposition of siderite. This in turn was followed by the formation of a little calcite. 

It appears that for the veins of the zinc-lead-silver-gold series the general sequence of deposi- 
tion was (1) pyrite and sphalerite with probably some galena, (2) galena, (3) siderite (or other 
iron-bearing carbonate) with small quantities of pyrite and sphalerite, and (4) calcite or barite 
(rare and nowhere abundant). While the foregoing represents the general order of crystal- 
lization, there was not an altogether rigid adherence to it. Probably one stage overlapped upon 
another. ^Moreover, there is fair evidence that after the first sulphides were found these were 
modified in many indistinguishable epochs by fracturing followed by solution and redeposition 
of sulphides. Thus the parts of the veins now worked are probably the result of a complex^ 
process of growth during which fracturing, solution, and deposition were many times repeated 
and in which ordinary ground water was an active agent. 

With reference to the stockworks and veins of the gold-silver-lead series the data for estab- 
lishing a sequence of mineralogical deposition are wanting. Presumably, however, the order of 
•deposition was generally similar to that deduced for the zinc-lead-silver-gold veins. 

164 



EFFECTS OF VABIOUS WAISL ROCKS ON ORE DEPOSITION. 165 

little is known of the nature of the Famcomb Hill veins prior to their oxidation. Specimens 
show that gold has crystallized with galena, sphalerite, and calcite, but the large masses of gold 
found have all been in oxidized material. 

The auriferous veins in the pre-Cambrian rocks show no recognizable depositional order in the 
aggregation of their constituents, so far as could be seen in the workings examined in 1908 and 
1909. Of the original mineralogical character of the blanket deposits little can now be learned. 

EFFECTS OF VARIOUS WALL ROCKS ON ORE DEPOSITION. 

That the character of the ore deposits depends to an important extent on the kind of rock 
inclosing them is apparent after even a cursory examination of the district. The blanke t 
deposits are foimd only in the bedded rocks of the Dakota and ** Wyoming" formations; cer- 
tain pocke ty accumulations of oxidized gold-silver ore are characteristically associated with 
the hard quartzitic fieds of the Dakota; the veins of the zinc-lead-^ver-gold-series are all in or 
near monzonite porphyry, while the stockworks and veins of the gold-silver-lead series are all 
intimately associated with the quartz monzonite porphyry; finally, the gold-bearing veins of 
Famcomb Hill suggest a more than accidental connection between them and the Upper Cre- 
taceous shale. 

It is in some cases difficult or impossible to determine what significance should be attached 
to a given association of a certain type of ore body with a particular rock. The question is 
likely to arise, especially if the kind of deposit under discussion is found only in one locality, 
whether the character of deposition would have been essentially different had another country 
rock happened to be prevulent at that place. Thus it may be asked. Do the characteristic 
features of the Famcomb Hill gold veins depend on the shale; or would they have been essen- 
tially the same were the hill all porphyry? If similar deposits were known elsewhere in the 
district and were invariably associated with the shale, the supposition of genetic connection would 
be manifestly much strengthened. As it is, the single group of these veins must be closely exam- 
ined to ascertain whether the shale has played a dominant or negligible part in their format 
tion. Study of them shows that the physical properties .of the shale have had a controlling 
influence on fissuring and on the deposition or concentration of the gold. Thus the finely 
. laminated but firm and homogeneous shale parted under stress in such a way as to be trav- 
ersed by remarkably sharp, narrow, and regular fissures which in proportion to their widths are 
unusujdly persistent. Had the rock all been porphyry the stresses would probably have been 
reUeved by more irregular fissuring, as may be seen in the sills of Famcomb Hill and in the 
deposits of the Jessie type. The lamination of the shale permitted the minor slips along bed- 
ding planes, which dislocated the little veins and in connection with the regular original fissures 
1 provided almost ideal conditions for the concentration of the gold in the course of oxidation and 
erosion. How far the chemical composition of the shale aided in precipitating the gold is 
doubtful. The shale contains some finely divided carbonaceous material to which, with abun- 
dant minute crystals of pyrite'and a little magnetite, its dark color is due. The rock, however, 
when pulverized and heated in a test tube yields no distinct odor and no distillate of volatile 
hydrocarbons. Heated in an open tube the powder gives off sulphur dioxide from the pyrite, 
which is not visible in fragments of the shale but can readily be separated from the powdered 
rock by panning. Undoubtedly the oxidation of these disseminated dustlike crystals of pyrite 
had much to do with producing the brown tint which the miners recognize as favorable to the 
occurrence of a gold pocket. The carbonaceous and pyritic particles in the shale probably had 
some influence also in precipitating the gold either from the original metallizing solutions or 
from descending waters of meteoric derivation. 

To the deposition of some ores, on the other hand, the shale appears to be decidedly unfa- 
vorable. The veins of the zinc-lead-silver-gold series, so far as can be determined from under- 
groimd workings now open and from records, become of little or no value when they pass wholly 
into, shale. There was, however, no good opportunity in 1909 to study this change in detail. 
The same rule holds good with respect to the gold-silver-lead group and is well illustrated in the 



> 



166 GEOLOGY AND ORE DEPOSITS OF BBECKENRIDGE DISTRICT, COLORADO. 

Jessie and Hamilton mines. In those mines wherever the shale is reached the ore ends within 
a few feet."^ The Whitehead stope (see fig. 18, p. 145), in places fully 15 feet wide, was carried for 
a few feet across the contact into the shale, but the ore was found to change into a network of 
small pyrite stringers containing very little gold. These become less numerous as they extend 
into the shale, and finally the face of the drift shows only one fairly regular stringer of calcite, 
less than an inch wide, containing a little pyrite. The near-by shale also contains disseminated 
pyrite. Thus the wide zone of fi'acturing in the porphyry diminishes to a little veinlet not 
unlike some of the Farncomb Hill veins below their enriched zone. The reason for the superi- 
ority of the porphyries over the shale as a country rock for ores of low gold tenor containing 
galena and sphalerite is not altogether clear. Bulk of sulphide material is important in deposits 
of these types, and the porphyries probably have an advantage over the shale in their greater 
rigidity and brittleness, which favors the opening of larger spaces than would be produced by 
an equal stress applied to the shale. Then, too, t he po rphyries, especially when much fractured 
orcrushed, appear to be more susceptible than the shale to metasomatic replacement. These 
relations are suggestive of those observed in the Coeur d'Alene district in Idaho,^ where the 
great silver-lead dej)osits, due largely to metasomatism subsequent to fissuring, are in brittle 
sericitic quartzites, whereas the smaller gold-bearing veins are in dark argillaceous slates. 

No satisfactory explanation has been found for the association of the two principal varie- 
ties of porphyry with different types of ore deposit — types which appear in fact to be more dis- 
tinct than are the two porphyries themselves. Why should the more calcic monzonite porphyry 
be accompanied by ores generally poor in gold but rich in galena and sphalerite, while in the 
quartz monzonite porphyry gold and silver are the important constituents, galena and sphal- 
erite being accessory? If the difference in the ores were a matter of locality rather than of 
porphyry, we should expect to find the Wire Patch ore bodies similar in character to those of the 
Wellington mine, instead of more nearly resembling, as they do, those of the Jessie mine. Appar- 
ently there is sufficient physical difference in the two rocks to favor the production of compara- 
tively simple and large fissures in the monzonite porphyry and complex zones of small fissures 
in the quartz monzonite porphyry. Partly in consequence of this difference the veins of the ' 
quartz monzonite porphyry have been more readily acted upon by descending waters, and there 
has been a greater concentration of galena in their upper portions. It has been shown also that 
the monzonite porphyry is probably a little older than the quartz monzonite porphyry. Accord- 
ingly the former rock may have been fissured when the latter was intruded. ^ Thus the stronger 
and simpler fissuring of the monzonite porphyry may be a consequence of more vigorous and 
long-continued stresses in that rock than in the quartz monzonite porphyry. Tliis perhaps is 
in part the explanation of the failure of the veins in the Wellington mine to maintain their 
regular character where they enter the quartz monzonite porphyry in the eastern part of the 

mine. 

VERTICAL VARIATIONS IN THE ORES. 

To an important extent in this district, changes in country rock have caused abrupt changes 
in the character of a deposit along vertical lines. Such departures from uniformity have been 
discussed in the preceding section. Of present concern are the more gradual transitions which 
may occur without any corresponding variation in the inclosing rocks and apparently quite 
independently of wall-rock influences. 

The vertical range through which the Breckenridge ores can be studied is, as already amply 
shown, exceedingly short. Direct observation is at present restricted to depths of less than 
350 feet. For such data as bear on successive downward clianges in the ores it is necessary to 
depend almost entirely on the history of the nmies. The records are most of them significantly 
alike. For instance, the Lucky, Minnie, Ella, and Cincinnati mines, in the monzonite porphyry 
of Mineral Hdl, all had some galena or cerusite ore of shipping grade near the surface, but at 
various depths, not accurately ascertainable though probably nowhere exceeding 300 feet, 
this ore gave place to more pyritic material. Some of this was milled for a time, but ultimately 

> Ransome, F. L., and Calkins, F. C, Geology and ore deposits of the Cceur d'Alene district, Idaho: Prof. Paper U. 8. Geol. Survey No. 62. 1908. 



OXIDATION AND ENRICHMENT. 167 

the ore ceased to yield a profit under methods then in vogue, and work was abandoned. The 
country rock of these mines is generally uniform and presents no variation that can accoimt for 
the downward decrease in the tenor of the ore. 

The Laurium mine originally shipped an argentiferous lead ore, but now yields a concen- 
trating gold-silver ore from a level between 200 and 300 feet below the old surface workings. In 
the Wire Patch, Jessie, Hamilton, and Cashier mines galena appears to have been more abundant 
near the surface than in the deeper workings. The lower tunnel of the Country Boy mine shows 
hardly any galena, although this sulphide was abundant in some of the ore from the upper work- 
ings. The Sallie Barber mine had some galena and considerable cerusite above the 240-foot 
level, but below that level the quantity of lead in the ore is negligible. The Wellington mine, 
it is true, shows abundant galena on the Oro adit level, the deepest workings now open, but the 
proportion of pyrite and sphalerite is apparently greater on the whole than in the upper levels. 
The deepest ore seen in 1909, at the east end of the Oro level and perhaps 350 feet from the 
surface, was sphalerite, containing 38 per cent of zinc and 1 per cent of lead. Unfortunately, 
nothing could be learned at first hand of the ore in the levels connecting with the Oro shaft 
below the present main adit. Probably the galena extends lower hei'e than it does far in under 
^lineral Hill, and it is known that some good lead ore came from stopes worked through the 
shaft. In these deeper workings also the country rock changes and shale, generally an unfavor- 
able rock for the occurrence of lead-zinc ores, appears in places under the porphyry. 

Doubtless if complete records were obtainable of all the idle or abandoned mines, they would 
lai^ly be a repetition of the same story — partly oxidized lead ores near the surface and sphal- 
eritic or pyritic ores below. Even the imperfect data now available establish without much 
question the limitation of the lead ores to a zone ranging from 200 to 300 feet deep and corre- 
sponding roughly to the present strongly accidented topography. There may, of course, be 
exceptions to this apparent rule; the underground workings are not extensive enough to afford 
a basis for sweeping assertion. The statement made, moreover, is far from implying that there 
is no galena below a depth of 300 feet; but all available evidence indicates that in this district 
300 feet, or perhaps more safely 400 feet, is the limiting depth for the occurrence of considerable 
bodies of essentially galena ore as opposed to sphaleritic or pyritic ores. 

This superficial accumulation of lead ores appears to be explainable by only one process — 
that of downward enrichment in connection with the erosion and oxidation of the upper parts of 
the veins. To suppose it a feature of the original deposition of the ores would require the unten- 
able correlative supposition that the topography of the district has not changed since the veins 
were first filled. The placers, with their nuggets of gold, sphalerite, and galena, are clear evi- 
dence to the contrary, and if it were possible to restore the landscape as it was when deposition 
of the ores began, one familiar with the hills and valleys of to-day would doubtless find himself 
in strange surroundings. The explanation that the concentration of the lead with its associated 
silver is the result of enrichment is consistent with the close relationship of the lead zone with 
oxidation and with the general evidence already presented that the galena in the veins is, in 
great part at least, younger than the pyrite and sphalerite. Consideration of the process or 
mode of enrichment may naturally be taken up in connection with the subject of oxidation. 

OXIDATION AND ENRICHMENT. 

GBOT7ND WATEB. 

The present surface of the ground water in this district is probably very irregular, although 
less so than it was before the hills were pierced by adits. The heavy winter snow, which in places 
lingers into the autumn, and the copious summer showers saturate all rocks except those on 
slopes so steep that the water is rapidly drained into near-by ravines. Along the main stream 
valleys the water level is practically at the surface of the ground. At the Sallie Barber shaft, 
situated on a sharp ridge about 600 feet above French Creek, the original water level was appar- 
ently at about 200 feet depth. In the Juventa shaft, similarly situated, on the north side of 
French Gulch, oxidation is said to have reached a depth of 200 feet, which presumably was about 



168 GEOLOGY AND ORE DEPOSITS OF BRECKENBIDGE DISTRICT, COLORADO. 

the water level. Beyond the additional facts that more or less water issues from nearly all tun- 
nels not choked with ice, and that most of the abandoned shafts in the district, no matter at what 
elevation, provided that they are not directly drained by adils, are partly filled with water, the 
underground workings supply vely little information regarding the original water surface. 

OXIDATION OF LEAD-ZINO OBES. 

So far as can be determined from rather scanty data general oxidation of the ores rarely 
extends below a depth of about 200 feet ^nd some residual galena may persist nearly to the 
surfaced On the other hand, incipient oxidation, as shown by the development of a little spongy 
impure smithsonite, may penetrate to a depth of 350 feet/ Oxidation goes deeper in the per- 
meable ores composed chiefly of sulphides than in siliceous veins such as occur in the pre- 
Cambrian rocks at the headwaters of the Blue. 

The usual product of oxidation in all but the essentiauy auriferous veins is a porous or 
earthy mixture of lynonite and cerusite with more or less siliceous and aluminous impurity. 
Some of the best ore is a soft, heavy, claylike material, consisting chiefly of lead carbonate, 
and probably lead sulphate also, with a considerable proportion of silver. The cerusite is in 
few places well crystallized, and the silver is rarely visible. Wire silver was found, however, 
in the oxidized ore of the Juventa mine, near Lincoln, and in the old Liberty tunnel of the 
Wellington group. 

A notable feature of the oxidized ores is their general high content of lead and silver as 
compared with the sulphides beneath. In some mines this difference was so great that their 
owners after extracting ore profitably to the base of the oxidized zone found the sulphides of so 
low a grade that work was abandoned. Here and there the oxidized ores also show a noteworthy 
concentration of gold even where the sulphide ores below contain only negligible quantities of 
that metal. Thus the Helen mine, on the south side of French Gulch, had some gold ore near 
the surface, although the latest and deepest workings have exposed nothing but a little sphaleritic 
zinc ore. In the Juventa mine, which produced some good oxidized ore to a depth of 200 feet 
and was then abandoned, gold is said to have been found in the form of wires suggestive of 
those from the auriferous veins of Famcomb Hill. 

The occurrence of native sulphur in the oxidized ore of the Iron Mask mine has been noted 
on page 81. This is probably not ^^ infrequent product of the reduction of ferric sulphate to 
the ferrous form by coming into contact with sulphides, and it is found occasionally in other 
districts, usually with partly oxidized ores; but as a rule the native element is oxidized to 
sulphuric acid during subsequent stages of weathering. In some way not known the sulphur 
in the Iron Mask was protected from the usual oxidation. 

In ores consisting essentially of pyrite, sphalerite, and galena exposed to weathering, 
the galena, owing to the comparatively strong chemical bond between lead and sulphur, proves 
most resistant to the oxidizing agencies, and in this district, as in most others where the climate 
is not too dry and erosion is fairly vigorous, residual lumps of galena may remain close to the 
surface, although the other sulphides have been entirely decomposed. Sphalerite, according to 
some writers,^ is supposed to be more resistant than galena and the other common metallic 
sulphides, but this does not accord with the usual experience in the field. 

At Breckenridge the oxidized ores rarely contain any sphalerite and below the zone of 
general oxidation this mineral is one of the first to part with its sulphur to form smithsonite. 

SUI^PHIDE ENBICHlffENT OF LEAD-ZINC OBES. 

The absence of sphalerite from the oxidized ore is probably not altogether or even largely 
a matter of direct weathering. Apparently much of the sphalerite was removed from the 
upper parts of the veins and was replaced by galena in advance of the slowly descending belt of 
oxidation in the manner so clearly described by Van Hise.^ Thus three factors cooperate to 

1 See, for Instance, Emmens, S. n., Tbechemlstiy of gossan: Eng. and Mln. Jour., 1892, pp. 582-583; also Beck, Richard, Erzlagerst&tten, 3d ed., 
Berlin, 1909, vol. 2, pp. 314, 328. Beck gives Emmens's arrangement of the sulphides in order of supposed affinity for sulphur and also presents 
Van nise's views, which involve a stronger affinity between lead and sulphur than between zinc and sulphur. 
Van Hise, C. R., A treatise on metamorphlsm: Hon. U. S. Oeol. Survey, vol. 47, 1904, pp. 1148-1151. 



OXIDATION AND ENBICHMENT. 169 

concentrate the lead in the upper parts of the veins — ( 1) the resistance of the galena to oxidation, 
(2) the comparative insolubihty of the sulphate and carbonate of lead, and (3) the readiness 
with which such lead as may go into solution in the form of sulphate or carbonate is acted on 
by sphalerite or pyrite and precipitated again as galena immediately below the zone in which 
oxidation is paramount. It is believed that a large proportion of the galena in the Brecken- 
ridge ores is the result of this downward concentration by atmospheric water, which, after 
percolating with comparative rapidity through the oxidized zone to the local ground-water 
level, thence moved more deliberately down through the sulphides, to emerge finally along the 
bottoms of the main valleys. 

The zinc abstracted from the oxidized zone should theoretically be carried down also and 
deposited as sphalerite by reaction with pyrite at a depth generally greater than that at which 
galena accumulates. Whether any large part of the sphalerite in tJie Breckenridge veins has 
been thus secondarily deposited is not clearly determinable from existing workings, although, 
as shown on page 164, there is certainly some sphalerite much younger than other. 

A large part of the iron originally present as pyrite remains in ihe oxidized ore as limonite, 
but some is doubtless carried down as bicarbonate, and together with the carbonates of calcium, 
magnesium, and manganese is deposited as an impure siderite in veinlets traversing the sulphide 
ores or as the lining of vugs in these ores. Additional iron is taken up by the ground water in any 
reaction involving the replacement of pyrite by sphalerite or galena, so that any of this water 
issuing as springs after performing its work of enrichment is likely to be strongly ferruginous, 
as is that of a spring near the Puzzle and Ouray mines. The iron-bearing water that flows from 
some old tunnels may have obtained some of its iron in this way, althougn probably most of it 
is merely the result of the oxidation of pyrite. 

In the processes connected with oxidation and enrichment the silver generally keeps close 
to the lead. Gold, so far as there is any evidence of its concentration in the lead-zinc ores, 
appears to accumulate in the zone of oxidation. 

OXIDATION AND ENBICHMENT OF THE FABNOOMB HTLL VEINS. 

Immediately in advance of oxidation the veins of Famcomb Hill consisted of pyrite, chal- 
copyrite, sphalerite, galena, calcite, and gold in various proportions. Some wire gold has been 
found in the unoxidized vein material, but not far below the zone of general weathering, and the 
principal concentration of the native metal was unquestionably connected with oxidation. 
Two intimately related processes appear to have been effective in enriching these veins. These, 
in the order of their action at one place, were (1) enrichment by solutions depositing calcite, 
galena, gold, and perhaps sphalerite below the zone of oxidation and (2) enrichment in the zone 
oTpxidation by solution and redeposition of the gold. 

It is clear that during the weathering of these veins the gold was acted on by some very 
efficient solvent, for otherwise it would be impossible to account for the large crystalline masses 
of gold characteristic of the hill. These could not have been deposited in the veins as part of 
their original fillings, for they are limited to the oxidized zone, and the once very productive 
placers below the hill show that this zone can not coincide with originally rich upper portions cf 
the veins. Evidently the original tops of the veins have been eroded away and their contained 
gold in part has been strewn along the ravines and down the main valleys, and in part has seeped 
down in solution along the fissures and been deposited in segregated masses. Active as solution 
must have been, erosion apparently was overtaking it; at least the richness of the placers 
proves that the gold was not carried down and redeposited fast enough to escape the forces of 
mechanical disintegration. 

As is well known, gold is a chemically inert substance, and ordinarily in the weathering of 
veins remains behind in the gossan or undergoes a merely mechanical concentration in conse- 
quence of the tendency of the heavy particles to work down to the bottom of any loose material. 
In some veins, however, the metal goes into solution and local placers or auriferous gossan may 
be lacking. Various substances or combinations of substances are probably effective in dis- 
solving gold in an oxidizing vein, and F. W. Clarke ^ gives a useful digest of experimental work 

1 The data of gfwclieinistry: Bull. U. S. Oeol. Survey No. 330, 1906, pp. 557-^558. 



170 GEOLOGY AND OBE DEPOSITS OF BBECKENBIDGE DISTBICT, COLOBADO. 

along this line. The efficacy of a dilute mixture of sulphates, chlorides, and manganese dioxide 
as absolvent of gold was long ago demonstrated by Richard Pearce/ who pointed out the appli- 
cability of such a mode of solution to explain the concentration of gold at the base of some 
gossans. Pearce, it is of interest to note, used native gold from the Ontario mine in his experi- 
ment. Some years later T. A. Rickard ' obtained similar results, and recently W. H. Emmons ' 
has called renewed attention to the possible influence of manganese dioxide in promoting solu- 
tion of gold during the oxidation of a- deposit and has collected considerable evidence in favor 
of this view. A. D. Brokaw,* working on the problem from the experimental side, has con- 
firmed the conclusion of H. N. Stokes * as to the insolubility of gold in ferric sulphate alone 
at ordinary temperature and the results obtained by Pearce and Rickard with manganese 
dioxide. Brokaw concludes that mixtures of ferric sulphate, sulphuric acid, and sodium chlo- 
ride in the strengths found in mine waters will readily dissolve gold in the presence of manga- 
nese dioxide, that free hydrochloric acid is more effective than ferric chloride in a solvent mix- 
ture, and that the presence of ferrous salts does not seriously diminish the solvent power of the 
solutions, provided manganese dioxide is present. In other words, ferrous salts under these 
conditions are unstable and quickly become ferric salts. 

Inasmuch as the veins of Famcomb Hill contain sphalerite and as a manganiferous car- 
bonate occurs in the Wire Patch mine in the same hill, it was thought that possibly the remark- 
able concentration of gold in these veins might be due to the action of solutions containing 
sulphates and chlorides in the presence of manganese dioxide, but tests of partly oxidized dark 
sphalerite from the Silver vein showed only a minute quantity of manganese, and some oxidized 
material from the Reveille vein showed none. Nor is it likely that in this elevated and well- 
watered region chlorides were ever abundant in the waters along the veins. Accordingly, 
while it can not be said that the veins are altogether free from chlorides and manganese dioxide, 
it does not appear that these constituents were ever so abundant as would accord with the 
supposition of their being an essential factor in the concentration of the gold. 

On the other hand, attempts to single out that essential factor have been unavailing. All 
or nearly all the veins contain a Uttle copper, and there is a suggestion that the presence of the 
salts of this metal may have had something to do with the solution of the gold, especially as 
well-developed wire and flake gold was observed some years ago implanted on oxidized copper 
ore in quartzite in the Globe district, Arizona;* but cupric sulphate, according to Stokes, has 
no appreciable solvent power up to 200° C, and it is doubtful whether the carbonates would be 
any more effective. 

Whatever may have caused the solution of the goU, the conditions for its deposition in 
rich pockets were almost ideal. Solutions traveling down the narrow fissures met others work- 
ing slowly down the dip of the shales, particularly along bedding slips and along the surfaces of 
the sheets of porphyry. ^ Thus waters carrying gold mingled at certain places with others 
depleted of the oxygen with which they started and possibly carrying more or less organic 
reducing matter picked up from the shale. The most important reducing and precipitating 
agents, however, were probably the sulphides 'already present in the veins and the pyrite finely 
disseminated through the wall rocks. There is a strong suggestion from the shape of some of 
the gold masses that their deposition began along the cleavage planes of calcite, opened prob- 
ably by the corrosive action of descending acid waters. Still other masses appear to have 
formed, in part at least, below the direct effects of oxidation, their deposition being accom- 
panied by that of calcite, galena, and sphalerite. The masses of gold show from their structure 
that each original particle of the metal became a nucleus upon which accumulated more of the 
gold drawn from the solutions by the attraction of the growing mass until finally, through the 
extension and coalescence of the smaller bodies, there resulted the branching crystal-coated 
forms for wliich the hill is famous. 



1 Address of retiring president: Proc. Colorado Sci. Soc., vol. 2, 1885-1887, p. 3. See also Trans. Am. Inst. Min. Eng., vol. 22, 18M, p. 739. 
s Trans. Am. Inst. Min. Eng., vol. 26, 1896, p. 978. 

• Outcrop of ore bodies: Min. and Sci. Press, Dec. 11, 1909, p. 782. 

« The solution of gold in the surface alterations of ore bodies: Jour. Geology, vol. 18, 1910, pp. 321-326. 

« Expoiments on the solution, transportation, and deposition of copper, silver, and gold: Econ. Geology, vol. 1, 1906, p. 650. 

• Ransome, F. L., Geology of the Globe copper district, Arizona: Prof. Paper U. 8. Geol. Survey No. 12, 1903, pp. 164-156. 



GEOLOGY AND OBE DEPOSITS OP BRECKENBIDGE DISTBICT, COLOBADO. 171 

GENESIS. 

To the investigator no phase of the geologic study of a group of ore deposits is more impor- 
tant or attractive than the problem of their origin, and there are few mining men so engrossed 
in the business of profitably adjusting tenor, tonnage, and treatment as to be without some 
interest in the same inquiry. The most absorbing questions, howevier, are generally those most 
difficult to answer satisfactorily and the reader who has followed the preceding descriptions 
scarcely need be told that the conditions at Breckenridge are not those likely to illumine with ,. 
much additional light the dusky comers of ore genesis. 

In a general way the principal ore-bearing fissures were probably initiated and perhaps 
controlled in direction by the great east-west stresses that accompanied the conspicuous post- 
** Laramie ^^ di stur bance oj^the Cordilleran regQii. This accords with the conclusion reached 
by Spurr and Garrey * for the near-by Georgetown quadrangle and there are no facts derivable 
from the Breckenridge district that suggest any other hypothesis. During this period of dias- 
trophism, which, although geologically brief, nevertheless may have had considerable duration 
measured by ordinary time units, the rnonzonitic porphyries were intruded and this igneous 
activity appears to have been followed by the faulting along the west sides of the Tenmile and 
Front ranges that has separated the sediments of the Tenmile district from those of the Breck- 
enridge district and has brought the Upper Cretaceous shale east of Breckenridge against the 
pre-Cambrian. So far as study of the Breckenridge area goes there is nothing to show that 
the porphyry was not intruded during or after the faulting. The assignment of priority to the 
intrusions rests entirely upon the work of Emmons' in the Leadville and Tenmile districts. 

The ore-bearing fissures, as has been shown, are not important elements of the regional 
structure and were probably opened by minor or secondary stresses. In the study of the Breck- 
enridge area the temptation to bring them into relation as subsidiary fractures with the great 
north-south dislocations represented by the Mosquito fault and by the Mount Guyot fault 
zone was strong. But Emmons has determined the Leadville and Tenmile ores to be older than 
the Mosquito fault, and the investigation of the small Breckenridge area has afforded no ground 
for questioning his conclusion. It is probable, however, that the movement along the Mosquito 
and Mount Guyot faults has been recurrent and consequently it is difficult to be sure that the 
faulting may not have begun before the deposition of ore and have continued after it perhaps 
up to the present time. With each slip along these principal lines of weakness there would 
probably be renewal of movement along the ore-filled fissures and at times the development of 
cross faults such as that which has offset the Siam and Iron mines. Some fissures, such as those 
of the Jessie mine, may have been formed by very slight movement, perhaps nothing more than 
a mere shpping or settling of the porphyry intersected by them upon the generally underlying 

shale. 

However formed, the fissures were filled primarily with pyrite, sphalerite, and galena, the 
sulphides containing more or less gold and silver. Quartz was deposited also in the veins in the 
pre-Cambrian rocks but is almost entirely lacking from the deposits in the porphyries and 
sedimentary rocks. 

That this deposition was connected in an essential way with the prophyry intrusions which 
it followed closely in time is beyond question. Emmons ' some years ago pointed out the close 
association of porphyries and ore in central Colorado, and Spurr and Garrey,* in their work on 
the Georgetown quadrangle, have recently enlarged upon this significant relationship. 

In spite of the generally feeble contact effects of the monzonitic intrusions in the Breck- 
inridge district, the invading rock has in some places changed the adjacent sedimentary rock to 
an aggregate consisting mainly of garnet and epidote, and the sporadic occurrence in several 
parts of the district of masses of similarly altered rock, many of them associated with abundant 
sulphides and some of them with specularite, indicates that at one time the rocks were penetrated 
by hot solutions capable of producing some of the changes characteristic of contact meta- 

« Prof. Paper U. 8. Oeol. Survey No. 63, 1908, p. 118. 

> Emmons, 8. F., Hon. U. S. Oeol. Survey, vol. 12, 1886; Tenmile special folio (No. 48), Oeol. Atlas U. S., U. S. Oeol. Survey, 1896. 

> Tenmile special folio (No. 48), Oeol. Atlas U. 8., U. S. Geol. Survey, 1S98, p. 6. 
« ProL Paper U S. QwL Survey No. 63, 1908, p. 129-135. 



172 GEOLOGY AND OBE DEPOSITS OF BRECKENBIDGE DISTRICT, COLORADO. 

morphism. There is no definite line to be drawn between the development of these metamorphic 
siUcates and the deposition of sulphide ores, although where silicates and sulphides occur together 
the sulphides are, in part at least, slightly younger. 

Whence came the various ore constituents ? Consideration of this question may be intro- 
duced by an attempt to gaia as clear a picture as possible of the ore-bearing fissures as channels 
for moving solutions. 

It has been shown in the preceding pages that individually the veins are not traceable for 
very great lengths and that some of them obviously die out along the strike/' Some years ago, 
in the discussion of a region where fissure veins are remarkably numerous and well exposed,* I 
called attention to the probability that the vertical range of a fissure is not necessarily so great 
as to be practically infiidte but that, before truncation by erosion, the length and depth of the 
fissure were roughly equal, or at least bore some simple proportion to each other. That some 
fissures die out at the depths reached in mining in other districts, more thoroughly developed 
than Breckenridge^ is a well-established fact and there appears to be little groimd for supposing 
that the comparatively short, deeply eroded fissures at Breckenridge, opened originally with no 
great displacement, continue downward with undiminished width for many thousands of feet. 
Such an idea is obviously imtenable with regard to fissures like those in the porphyry of the 
Jessie xnine or those associated with some of the pockety occurrence of gold in quartzite. The 
Famcomb Hill veins also can scarcely be thought of as retaining their identity for many himdred 
tfeet below the level of the Fair txmnel. Much more reasonable, especially in view of the different 
Ikinds of coimtry rock that in this district would have to be traversed "by most veins having great 
^ertical ranges, is the supposition that the vein fissures have never extended down as direct, 
open, and continuous channels to a vaguely imaginable magmatic source but that they pinch 
or split into small branches at moderate depth and finally lose themselves in the minute irregular 
fractures from which no rock in a disturbed region is wholly free. kIji other words, for the con- 
ception of a single, simple profound cleft up which solutions from magmatic or other abyssal 
sources could freely and rapidly ascend is substituted a different picture, that of fissures extend- 
ing only to depths of a few hundred or a few thousand feet, there dying out and linked perhaps 
by small fractures with other fissures, which also in turn die' out, the general tendency of the 
complex system being toward smaller and less numerous fissures with increase of depth and the 
whole being complicated by more or less later faulting. According to this view, few if any 
veins in the Breckenridge district were filled by the direct ascent of solutions from below. The 
fissures now occupied by the veins were, it is believed, semi-isolated spaces, partly filled with 
crushed or shattered rock into which the ore-depositing solutions with their contained gases 
penetrated at many points after pursuing devious coinrses tlnrough other fissures and through 
rock rendered porous by xninute fractiures, such as those described on page 56, or possessed of 
this property by virtue of original texture. K this view is substantially correct it follows that 
the solutions, whatever may have been their origin, deposited their load only after they had 
exceptional opportunity to search the solid rocks through which they passed and to extract from 
these rocks such constituents as they could dissolve. The next questions are. What were these 
solutions to start with and what did they gather on their journey ? 

It is clear that the ore-bearing, solutions carried large quantities of sulphur, sufficient not 
only to fill the veins with sulphides, but to impregnate the country rock extensively with pyrite 
and less abundantly with sphalerite and galena. Of all the constituents, this sulphur, probably 
in the form of hyclrogen sulphide, can be most confidently assigned to magmatic sources. It has 
been shown also, in Chapter VII (pp. 93-102), that the solutions were rich in carbon dioxide, or 
bicarbonates, and it is highly probable that carbon dioxide with hydrogen sulphide and water 
issued from the deep-seated cooUng masses of monzonitic magma from which the visible por- 
phyry sheets are presimiably offshoots. The sheets and small intrusions as they solidified must 
themselves also have contributed a share of these constituents. While' most of the carbon 
dioxide is supposed to have been of igneous origin, it is to be remembered that, as A. B6champ * 



1 Ransome, F. L., Eoonomic geology of the Silverton quadrangle, Colorado: Bull. U. S. Oeol. Survey No. 182, 1901, p. 62. 
s Recherobes sur Tdtat du souiro dans les eaux minfimles sulfurfes: Annales chlm. phys., 4th ser., vol. 16, 18G9, p. 202. 



GENESIS. 173 

long ago showed; hydrogen sulphide in solution will dissolve limestone, producing the soluble 
calcium hydrosulphide (Ca(SH)2) and calcium bicarbonate (HjCaCCX),),). Thus a hydrogen sul- 
phide water in passing through rocks containing alkali-earth carbonates becomes carbonated. 

'^ejjalljockaltgrjy^ discussed in Chapter VII (pp. 93-102) indicates that the solutions 
when thejnreached the place of ore deposition, especially in the monzonite porphyry, contained 
in abundance the bicarbonates of iron, magnesium, manganese, and calcium; for the brown or 
sideritic carbonate developed in the porphyry, as contrasted with that filling cracks in the ore, 
appears to be connected with the original deposition rather than with any reworking of the vein 
materials by descending surface waters, although it must be admitted that deeper mining would 
probably afford much more satisfactory evidence on this point. The smirce of the constituents 
ofjtbe .sideritic carbonate is not known with certainty, but they are believed to have been leached 
from the solid rocks by the hot carbonated solutions on their roundabout journey to the fissures 
in which the ores were deposited. Recently F. Henrich * has studied experimentally the 
action of carbon dioxide in aqueous solution on finely pulverized rocks. He foimd that even at 
ordinary temperatures and atmospheric pressure a notable proportion of the rock went into 
solution, the loss in a basalt being nearly 3 per cent. According to Henrich, no constituent of 
the rocks wholly escapes attack, but those most readily extracted are manganese, calciimi, and 
iron. In rocks where it occurs in important quantity magnesium also is readily removed. 
In view of these results it appears very unlikely that thermal carbonated solutions, necessarily 
under considerable pressure, could soak their way through many small openings in the rocks 
without robbing these of precisely the constituents that were subsequently introduced into the 
wall rocks of the veins. Such solutions would extract most from the femic and calcic rocks, 
and this in part explains the pronounced carbonatization associated with the deposition of ores 
in and near the monzonite porphyry and the very slight development of carbonates in the 
siliceous quartz monzonite porphyry or in the pre-Cambrian rocks. Presumably the original 
characters of the altered rocks have also much to do, in a receptive way, with the differences in 
wall-rock alteration, the introduction of carbonates being more easily brought about in those 
rocks, already rich in primary calcic and femic minerals, that yield on decomposition the bases 
to combine with carbonic acid as comparatively insoluble compoimds, whereas in siliceous 
potash-bearing rocks the effect of the carbon dioxide is most manifest in the development of 
sericite and the removal of the alkali earth bases. Although the ore-bearing solutions are 
thought to have obtained their carbonate bases from the rocks, it is not supposed that they were 
derived from the inmiediate wall rock of the present veins. If so, there might be expected to 
intervene between the monzonite porphyry to which carbonates have been added and the 
nearly fresh rock a few feet away from the veins a variety from which iron, magnesium, calcimn, 
and manganese have been removed. No such change has been noted. On the other hand, 
the rock showing an addition of ferruginous carbonate appears to pass gradually into nearly 
fresh rock, in which, however, as described on page 66, there are microscopic fractures filled with 
carbonates. It is probable that in part through such minute channels the bases of carbonates 
were gathered at various distances from the veins, the loss of these constituents in a rock at 
any one place being probably too small to greatly change its appearance or character. 

The Breckenridge district presents no decisive evidence as to the source of the metals 
combined with sulphur in the veins. Much of the pyrite in the wall rocks was formed by the 
action of hydrogen sulphide on the iron originally combined as silicates or magnetite; but the 
pyrite in the veins with the accompanying sphalerite and galena has been brought from some 
distance. There is nothing in the occurrence of these sulphides that denies them an origin as 
magnetic emanations. On the other hand, the way is equally open for anyone to regard them 
as having been leached by thermal and gas-charged waters from the solid rocks. The latter 
speculation has in its favor, first, the consideration that thermal waters could hardly have 
made their way through the rocks by the circuitous routes supposed without gathering con- 
siderable material on the way, and, second, the association of certain kinds of deposits with 

^ iibet die Rinwlrkung von kohlensAurehaltlgen Wasser auf Q«BteiDe und Ober den Uispnuig and dem Mechanismus der KohleDsftuiefOhienden 
Tharmen: Zeltsohr. piakt. Gwlogto, vol. 18, 1910, pp. 86-M. 



174 GEOLOGY AND OBB DEPOSITS OF BRECKENRIDGB DISTRICT, COLORADO. 

particular kinds of country rock, thus suggesting that the adjacent rock in each case had some- 
thing to do with determining the contents of the deposit. Yet it can scarcely be supposed 
that the metallic constituents of an ore deposit like that of the Jessie were derived entirely 
from the comparatively small body of porphyry within which they lie. On the wholtf^ it appears 
that the local evidence available is insufficient for an opinion of any real value.*^ It is quite 
possible that each of the metals may in part have been given off from cooling magma and in 
part collected from the solid rocks. In their study of the Geoi^etown and Idaho Springs areas 
Spurr and Garrey ^ concluded that whereas the earthy gangue minerals and the iron and man- 
ganese of the veins were derived from the country rock, the more valuable metals — gold, silver, 
copper, lead, zinc, arsenic, and antimony— were given off from solidifymg magma. They 
based their conclusion mainly on the circumstance that they were able to distinguish two 
groups of deposits, each being related to a different set of intrusive rocks, the argument being 
that if the metals were all derived from the solid rocks by leaching the ore deposition would 
have been of the same character after each period of eruptive activity. This, however, is 
perhaps not entirely conclusive, for the difference may have inhered in the solvents given off 
from the two magmas, enabling each to make a different selection of metals in its journey 
through the rocks. Moreover, rocks subjected to one leaching might not yield constituents in 
the same proportion on a second leaching. As these authors remark: 

If the evidence afforded by either of these vein groups was taken separately, the hypothesis that these rarer metals 
[gold, silver, etc.] have been derived from the hot ascending magmatic waters from the rocks through which these 
waters ascended is as fully justified as that which supposes the metals to have been given off from the magma along 
with the mineralizing agents, since the crystalline igneous rocks through which the waters passed contain some, if not 
all, of these metals in small quantities. 

In the Breckenridge district there is no representation of the second or alkali-rich magma 
which in the Idaho Springs district followed the monzonitic magma and no evidence of two 
distinct periods of ore deposition. At the present time the district contains no thermal springs. 

AGE OF THE DEPOSITS. 

As the deposition of the Breckenridge ores, part of which are in Upper Cretaceous 
shale, followed the post-* 'Laramie" diastrophism) they are unquestionably of Terjiary age. 
They were deeply eroded before the glacial epoch and probably were deposited in the early 
part of the Tertiary. During later Tertiary time erosion and enrichment were at work and a 
layer of rock ot unknown thickness was removed. The surface was probably reduced several 
thousand feet^efore the beginning of the Pleistocene. 

^ Prof. Paper U. S. Geol. Survey No. G3, 1908, p. 157. 



CHAPTER XIV. 
GOLD PLACERS. 

CLASSIFICATION. 

The placers of Breckenridge may be divided into three general classes — (1) the bench or 
high-level placers, (2) the deep or low-level placers, and (3) the gulch washings. The bench 
placers are those in the deposits described in Chapter VI (pp. 72-76) as terrace gravels and older 
hillside wash. They have been worked almost entirely by hydraulic methods. The deep placers 
are those in what have been described as the low-level gravels. They were formerly worked in 
part by bedrock drifting and imsuccessfuUy by hydraulic elevators but are now dredged. The 
gulch washings include placers of small area and generally of st^ep slope lying in the bottoms of 
minor gulches. Their material as a rule is soil and angular rock waste from the adjacent slopes. 
These were the first placers to be profitably worked in this district, the methods employed rang- 
ing from panning and rocking 'to booming and the use of high-pressure water jets from giants or 
monitors. These three groups of placers will be described in the general historical order of their 
exploitation as sketched in Chapter I (pp. 16-20). 

GULCH WASHINGS. 

The most noted gulch washings in the district are those of Georgia, American, and Dry 
gulches, on the north slope of Farncomb Hill. These were first worked in the early sixties, and 
their total yield has been variously estimated at $5,000,000 to $10,000,000. The material of 
these placers is imconsolidated soil and shale detritus, with scattered angular blocks of por- 
phyry, and attains a maximum thickness in the bottoms of the gulches of about 25 feet. The 
bedrock is generally shale, but this is cut by some dikes of porphyry, and porphyry is the general < 
bedrock along the northwest side of the placer workings of upper Georgia Gulch. The shales 
strike generally across the gulches and dip downstream, thus forming ideal natural riflSes for 
catching the gold. Moreover, they weather into small fragments such as are easily washed 
through the sluices and constitute a bedrock whose comparatively soft superficial layer, contain- 
ing more or less detrital gold, was easily removed by the miners. 

Work on these placers has long ceased, and little can now be learned of the methods used in 
exploiting them, of their tenor per yard, or of the occurrence and character of the gold. Pre- 
sumably pans and rockers were first employed, and there appears to have been some drifting 
practiced at the lower end of the gulch. Hydraulic washing was later introduced, and long after 
the rich alluvial material had been exhausted the dimips of some of the Farncomb Hill timnels, 
with much detritus from the hillside, were washed down over the old placer ground. According 
to all accounts much of the gold was coarse, and most of the nuggets showed the wiry and flaky 
texture afterwards found to be so eminently characteristic of the gold from the Farncomb Hill 
veins. There is no doubt as to the origin of the placer gold of these gulches. It came from the 
wonderful little veins in the shale of Farncomb Hill and perhaps partly also from' similar veins, 
of which the rich parts have been eroded away, in the shale of Himabug and Brewery hills. It 
had not been subjected to the pounding and wear of distant transport along a bowldery stream 
bed, but had crept gently down the slope with accompanying soil and rock fragments for a few 
hundred feet to the bottoms of the little ravines, where it gradually accimiulatea in a rich narrow 
layer or ribbon close to bedrock. 

Another deposit of note that probably belongs with this class is the Wire Patch placer, on 
the southwest slope of Farncomb Hill, a mile east of Lincoln. There is scarcely any well-defined 

175 



176 GEOLOGY AND OBE DEPOSITS OP BRECKENBIDGE DISTRICT, COLORADO. 

gulch here^ and the deposit is little more than a patch of shaly soil carrying many porphyry frag- 
ments that has accumulated on shale bedrock near the* base of the steep slope on which the Ele- 
phant ore body comes to the surface. The gold evidently came largely from the Elephant ore 
body and perhaps in part also from the veins of the Ontario mine. It is long since work was done 
in this placer, which is nearly exhausted, although there is said to be a little auriferous detritus 
of good grade at the head of the placer, under the Wire Patch miU. 

Some washing was done in Australia Gulch,' which opens on the south side of French Gulch 
about a mile west of Lincoln, but this was not nearly so productive a ravine as Nigger Gulch, 
farther west, where, on the Lillian Vale or T. H. Fuller placer, the material washed was soil and 
angular fragments of porphyry on a porphyry bedrock. The gold appears to have been derived 
from the disintegration of the Dunkin, Juniata, and Washington groups of veins on Nigger Hill. 

Gibson Gulch, on the south side of Gibson Hill, contained a small but rich placer deposit 
with shale and diorite porphyry bedrock. There was at one time a little settlement in the gulch, 
and the gold is said to have been obtained with pans and rockers. 

The bottoms of Illinois Gulch and its branches are pl*obably best classed as gulch placers and 
were long ago worked over by primitive hand methods. North of the gulch and east of the main 
tunnels of the Washington mine a large pit, shown by the contours on Plate I (in pocket), has 
been excavated by hydraulic mining in the Dakota quartzite. The rock in this vicinity is much 
fractured, probably in consequence of proximity to the fault shown in Plate I. The material 
washed consisted of angular blocks of quartzite, some of large size, mingled with earth, the 
whole resting with no sharp distinction upon a very rough bedrock of fractured quartzite. I 
was unable to learn how successful the operators of this placer were or when the work was done. 
The gold obtained had not traveled far and was probably deposited originally in the fissures of 
the quartzite. 

In the class of gulch placers, also, are included some shallow washings along the Blue south- 
west of Little Mountain. Real bedrock does not appear to have been reached in these pits 
and the material worked over by the early miners probably consisted for the most part of 
auriferous and argentiferous detritus that had slid or crept down from Little Mountain over 
the moraine. 

Many other examples of gulch placers might be cited and described, but their characters 
are so similar and all have so long been exhausted that further detail is scarcely necessary. 
Virtually every gulch leading down from known auriferous lode -deposits has yielded its quota 
to the total placer output. It can not be said, however, that the former productivity of the 
placers is directly proportional to the known importance of the respective lodes. Thus on the 
south side of the Swan the placers of Galena Gulch, in which no important mine has yet been 
opened, have been more productive than those of Summit Gulch, in which is the Hamilton 
mine, or than those of Browns Gulch, in which is the Cashier mine. Probably in this, as in 
other placer districts, much gold may have been contributed to the placers from veins too 
small or too uneven in tenor to stope or may have been supplied by the disintegration of rich 
shoots in veins of which only low-grade remnants have been left. 

The slope of the bedrock in the gulch placers is generally much steeper than the gradient's 
of large streams and some of the richest placer ground on Famcomb Hill lay on a slope of 1 
in 6. Retention of large quantities of gold on such a grade was probably favored by the 
angular form of the nuggets, by the laminated character of the bedrock, and by ^ more abundant 
supply of soil and rock detritus than could be scoured out by the small streamlets in the gulches. 
In these deposits soil creep may have contributed fully as much as flowing water to the 
concentration of the gold. 

BENCH PLACERS. 

The general distribution and character of the terrace gravels and older hillside wash have 
been described in Chapter VI (pp. 72-76). They have been extensively worked in the past by 
the ordinary hydraulic method, but large quantities of material, as may be seen from Plate I 
(in pocket), are as yet untouched, the bulk of gravel washed being probably less than 5 per 



GOLD PLACEBS. 177 

cent of the total volume of the deposits. The bench placers were actively worked up to about 
10 years ago, but since that time this form of mining has declined and in 1909 no hydrauUc 
work was in progress. The causes of the decline are chiefly the working out of the richest 
known areas up to the ditches that supphed the water for the giants. Although in some placers 
there is good gravel above the ditch the quantity is probably not sufficient to encourage anyone 
to construct a new high-level ditch many miles in length or to purchase water rights for the 
purpose. In other placers the increasing thickness of the gravels as they are followed in from 
the first exposure on the hillside and the greater cost of handUng the waste have turned the 
scale against a deposit that probably was nowhere of high grade. In one or two cases the 
hydrauUc miner has been enjoined by owners of low-level placers from washing d6bris over 
their property. 

It is noteworthy that although several placer pits have been opened in the terrace gravels 
along the Blue these placers have never been worked as extensively or as successfully as those 
on Gold Run and French Gulch near their mouths. 

The Peabody and near-by placers in the older wash of Gold Run (PI. XVI, p. 74) were prob- 
ably more persistently worked and more productive than any other group of bench placers in the 
district, their total output being generally estimated at about $750,000. The character of the 
material, angular detritus loosely held in a friable matrix, has been described on page 75 and 
is illustrated in Plate XVI, B, The maximum depth of ^he deposit is about 40 feet. The bed- 
rock is generally black shale, from which it was a comparatively easy matter to collect virtually 
all of the gold. 

The gold of the Gold Run placers was certainly not brought from a distance, but probably 
worked slowly down the slope, chiefly from a series of veins exploited in the Jumbo, Extension, 
and Little Corporal mines. The process of concentration was thus similar to that operative 
in the formation of the gulch placers. 

Another important group of workings is that generally known as the Sisler or Mekka 
placers, on the French Gulch slope of Nigger Hill, of which a general view is shown in Plate 
rV, 5 (p. 18). The material here differs from that in Gold Run in being terrace gravel, which 
contains well-rounded bowlders, many of them decomposed, rudely interstratified with yellow 
sandy clay. These gravels range from 10 to 20 feet in thickness, are readily broken down by 
water under pressure, and rest generally on decomposed quartz monzonite porphyry. The 
large bowlders, up to 3 feet in diameter, are as a rule in the lower part of the deposit, the upper 
part being more clayey and more distinctly stratified. 

Very httle information regarding the Mekka placers is now obtainable in the district. 
The roundness of the bowlders with which the gold is associated, however, suggests that in 
part at least it was brought down by a powerful stream from points farther up French Gulch. 

The last place at which hydraulic washing was practiced in the district was at the Banner 
placer (PI. XIV, B), on the west side of the Blue about a mile north of Breckenridge. The 
character of these gravels has been described on page 73 and is illustrated in Plate XV (p. 72). 

DEEP PLACERS. 
GENERAL FEATT7BES OF THE CHANNELS. 

The low-level gravels, as already noted in Chapter V (pp. 78-79), occupy the bottoms of the 
present valleys of the larger streams and attain economic importance along Blue and Swan 
rivers and in French Gulch. Their distribution has been described in the chapter referred to 
and is shown on Plate I (in pocket). The thickness of the gravel along the Swan and in French 
Gulch rarely exceeds 50 feet. Along the Blue the maximum depth of the bedrock channel ranges 
from 50 feet in the vicinity of Valdoro to about 90 feet near Breckenridge. Consequently, unless 
there has been some recent tilting of the region, the old stream, along this part of its course, 
flowed on a lower gradient than the present Blue River, which has a fall of about 100 feet to 
the mile between Breckenridge and Valdoro. About 2^ miles north of Breckenridge the 

90047°— No. 75—11 12 



178 GEOLOGY AND ORE DEPOSITS OF BRECKENRIDGE DISTRICT, COLORADO. 

greatest depth to bedrock, as shown by drilling, is 54 feet and near Braddocks it is 60 feet, the 
line of greatest depth lying east of the present river in this part of its course. 

The width of the gravel-filled valley bottoms ranges from 600 to 3,000 feet along the Blue, 
from 500 to 1,200 feet along the Swan, and from 700 to 1,500 feet in French Gulch. It thus 
appears that the gravel occupies comparatively broad-bottomed straths whose slopes merge 
without abrupt change of grade into those of the boimding hills. The general shape of these 
valleys in cross section is shown in Plate XXXII. Evidently the streams before the deposition 
of gravel began were well graded; they were no longer actively engaged in deepening their 
channels but were leisurely widening and smoothing them as if in preparation for future dredging 
operations. Drilling and dredging show that the rock bottoms of the channels are, on the 
whole, smooth, although at a few places where the streams have cut through quartzite that 
rock appears to be sufficiently rough, in connection with its hardness, to tax severely the machin- 
ery of dredges attempting to scoop up the auriferous surface layer of the bedrock. Shale as a 
rule is so soft as to be easily excavated by the buckets, and the porphyries, owing to more or 
less decomposition, also constitute a tractable bottom material. 

By no means all of the gravel is commercially auriferous. The workable strip, or ' 'channel,'' 
in the dredgers' pa^rlance, is generally from 180 to 400 feet wide and follows a sinuous course 
along the valley. It has no regular relation to the channel of the present stream nor does it 
invariably correspond to the deepest part of the bedrock trough. Its lateral limits are indefinite 
and irregular. Both they and the tenor of the deposit are in many places clearly aflfected by 
the proximity of the lateral gulches, which themselves yielded alluvial gold to the early placer 
miners. Thus the gravels of French Gulch are especially rich below Nigger Gulch and some 
of the best ground known on the Swan is at the mouth of Galena Gtdch. Some rich gravel 
occurs also on the Swan for some distance below the mouths of Georgia and American gulches, 
in which were the most productive shallow placers worked in the district. 

MATERIAL. 

The gravels are generally loose and facilitate dredging by the readiness with which they 
crumble when they are undercut by the buckets. They show some differences in this respect, 
however, those on the lower Swan caving more easily than those on French Creek. The chief 
obstacles to profitable exploitation are the coarseness and depth of the material. The groimd 
least amenable to dredging is that along the Blue, where the gravel is not only thicker than on 
the other streams, but contains a greater proportion of large bowlders. In the Gold Pan 
workings, close to the terminal moraine near Breckenridge, bowlders 6 feet across are by no 
means rare, those 3 feet 6 inches across are common, and those 18 inches to 2 feet are abundant. 
These are mostly of pre-Cambrian material, but many are of porphyry. All are more or less 
water worn and most are well rounded. A general view of these gravels as exposed in the walls 
of the Gold Pan pit and an illustration of some of the bowlders taken from the same excavation 
may be seen in Plate XVIII (p. 78). These large masses, weighing 4 or 5 tons, had to be 
swimg out of the pit with derricks and the expense of handling them contributed largely to the 
failure of the Gold Pan hydraulic elevators. The material is ill assorted, is not perceptibly 
stratified, and contains the large bowlders distributed from top to bottom. Lower down the 
river the size of the bowlders decreases, but those 4 feet in diameter are raised by the dredge 
near the mouth of the Swan and bowlders over 3 feet across are embarrassingly abundant. 
The gravels of the Swan below Swan City are not so coarse as those on the Blue and offer no 
difficulties to well-constructed modem dredges. On the upper Swan, near Georgia Gulch, the 
material is coarser than on the lower reaches of the stream. In that part of French Gulch 
where dredging is in progress, between Prospect and Nigger hills, the bowlders range from 3 
feet in diameter down, 18-inch and 2-foot ones being abimdant. They consist mostly of por- 
phyry and quartzite and are not so well rounded as those of the Blue, which have in great part 
been derived from the older bench gravels. 



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GOLD PLACERS. 179 

TENOB. 

The Breckenridge deep gravels, in comparison with some placers in the Lena Basin of 
Siberia, which, according to C. W. Purington,* average as much as $10 a cubic yard, are of 
decidedly low grade. Their tenor, owing to some local tendency toward exaggeration and to 
the usual reluctance on the part of those engaged in dredging or prospecting to publish the 
financial results of their operations, is not a matter of common knowledge. The statement is 
frequently made, for example, that the gravels of the lower Swan and of the Blue near Valdoro 
average about 20 cents and that the gravels of French Creek average from 50 cents to $1 a yard. 
Much higher figures are often given and it is not unusual to hear that most of the Swan grave] 
averages 50 cents or that certain areas of prospected but unworked ground will yield an average 
of $1 .50 a yard. In the prospectus issued a few years ago by the Gold Pan Mining Co. the average 
tenor of the Blue River gravels just south of Breckenridge was stated to be about 60 cents, which 
is clearly far above the mark. 

Undoubtedly some rich spots are occasionally found where the gravel does carry over a 
dollar a yard, but extensive bodies of material averaging as much as 50 cents a cubic yard are 
certainly very rare. In French Creek one dredge working in unusually good ground below the 
mouth of Nigger Gulch excavated in 1909 an area of 10 acres on the Mekka placer and the gravel 
averaged 42 cents a yard. The other dredge in the same gulch is credibly reported to have worked 
some gravel carrying from 60 to 70 cents a yard, but the average is undoubtedly under 50 
cents. A few years ago W. W. Dyar, who in 1902-1904 was general manager of the American 
Gold Dredging Co., bored a line of drill holes across French Gulch near its mouth and obtained 
an average of 25 cents a yard from a pay channel 200 to 300 feet wide and averaging about 30 
feet deep. 

The gravels on the Blue are undoubtedly of lower grade on the whole than those worked on 
French Creek. The ground already dredged on this stream near the mouth of the Swan is known to 
have had an average tenor considerably under 20 cents. Between a point about 2^ miles north of 
Valdoro and French Gulch the channel has been tested by 11 transverse lines of drill holes 
whose records were obtained in connection with this report.* The highest rate per yard indicated 
by any hole (calculated from outside diameter of the shoe) was 61 cents, just north of the con- 
fluence of the Blue and the Swan. A mile farther north the highest tenor found was 42 cents. 
Upstream from the mouth of the Swan the highest rate calculated in the series of holes here 
considered was 27 cents, just north of the highway bridge, 2 miles down stream from Breckenridge. 
The average of the best hole in each of the 1 1 lines is 23 cents and this is probably not far from 
the average tenor of the richest line along the old channel. This average would be materially 
reduced by taking into consideration those holes on both sides of this line that would be included 
in the workable width of the channel. Out of the 237 holes here discussed, there are 47 that give 
returns of 5 cents or over. The average of these is a little over 12 cents. 

On the Swan the data from 15 sets of drill holes have been examined. The highest assays 
recorded are $1.49 a cubic yard just above the mouth of Summit Gulch and $1.19 a yard on the 
middle fork near Snyder's (?amp (bench mark 9823). The average of the best hole from each 
group is a little over 30 cents. Out of 96 holes, 46 afford returns of 5 cents or over and the aver- 
age of these 46 is about 17 cents. If the two exceptionally rich holes mentioned above are 
omitted the average becomes a little less than 12 cents. 

Such averages as those just given obviously do not represent the actual average gold con- 
tents of the gravels. The results of drill tests are more or less uncertain at best and differ mate- 
rially with differences in the methods of calculation used to transform the assay returns from the 
drill sand into terms of value per cubic yard. The positions of the holes also should be taken 
into account and their records properly weighted in order to obtain from them a fair average of 
the whole. But notwithstanding these elements of inaccuracy the figures as given serve to 
indicate in a fairly definite way the general tenor of the deposits. It is scarcely necessary to 
note that all statements as to value per yard take into account the whole thickness of gravel 

1 Mln. Mag., vol. 2, 1910, p. 208. * There are also other holes whose logs are not available. 



180 GEOLOGY AND ORE DEPOSITS OF BRECKENBIDGE DISTRICT, COLORADO. 

from surface to bedrock. Of course if barren overburden were eliminated very much higher 
returns than those given would be obtained in many places. 

As a rule the gold is not evenly distributed through the gravel from top to bottom, but is 
concentrated in the lower 6 feet or in the decomposed surface layer of the bedrock. There are a 
few exceptional places where nearly the whole thickness of the gravel is auriferous. One of 
these is at the junction of the Swan and the Blue. A similar distribution of the gold is said to 
characterize some of the gravel near Dillon, north of the area here particularly studied. 

DBIIiLING. 

It was not until about 1902 that chum drills began to be used for prospecting the gravels. 
Exploration prior to that time was by the troublesome and expensive method of sinking shafts 
to bedrock through the loose and saturated gravel. During the last 15 years the Blue north 
of French Gulch and the Swan from its mouth to Georgia Gulch have been systematically 
prospected by transverse lines of drill holes, the holes on each line being generally 100 feet 
apart. Some of the older holes, however, are not in lines, but were bored at isolated places or 
in random groups. 

Some drilling has been done also in French Gulch, especially on the Mekka placer of the 
French Gulch Dredging Co., but until the year 1909 very little systematic drilling was done on 
French Creek and there are few lines of holes that extend entirely across the valley. 

In the vicinity of Breckenridge the Gold Pan Mining Co. put down 30 or more drill holes 
on the Maggie placer, some of them north of the terminal moraine and some of them along the 
river where it has cut through the glacial material. The same company also did some drilling 
farther up the Blue, in the vicinity of the Goose Pasture. The results of these tests, however, 
could not be ascertained in 1909. 

Ordinary portable Keystone drilling machines are used in prospecting. The methods 
employed are not altogether uniform. Some drillers advance the bit about a foot ahead of 
the casing, which is then driven down. Others drive the casing ahead* of the bit wherever 
possible. By some samplers the sand is all put through a small rocker, the concentrates only 
being panned. By others all the material is panned. In 1909 one piece of ground was being 
appraised by panning the drill sands, counting the colors in the pan, and estimating therefrom 
the value of the gravel per yard. The Colorado Gold Dredging Co., on the other hand, accepts 
no such guesswork. The concentrates from the pans are treated with nitric acid and the gold 
is then amalgamated, ignited, and weighed. 

The accuracy of drill prospecting evidently depends largely on the closeness with which 
the volume of the drill hole can be calculated — that is, the exact original volume of the column 
of gravel yielding a certain number of milligrams or centigrams of gold must be known in order 
to calculate from those data the number of cents per cubic yard of gravel. Two methods are 
ordinarily used. One consists in actually measuring the volume of the sand pumped from the 
hole. The other consists in calculating the volume of the hole on the supposition that it cor- 
responds to the outside diameter of the shoe on the .end of the casing, usually 7^ inches. In 
the records of the Colorado Gold Dredging Co. the results by both methods are given and are 
often at variance. As a rule, the results by the method of measured excavation are higher, 
especially in the older records. One of the earlier holes, for example, gave 4.6 cents a yard 
when the outside diameter of the shoe was used and 25.4 cents by the other method. According 
to Mr. W. H. Lohman, manager for the company, the method in which the diameter of the 
shoe is used is the more reliable and agrees more closely than the other with the results later 
obtained by dredging. In this report all statements regarding the tenor of gravels as deter- 
mined by driUing are based on the method of calculation wherein the outside diameter of the 
shoe is used. 

DBBDGING. 

The history of the development of dredging in this district has been given on pages 19-20. 
In 1909 there were four boats in successful operation. Near Valdoro the Colorado Gold Dredg- 
ing Co. has two Bucyrus dredges of identical size and capacity. One, shown in Plate XXXIII, 



J. S. QEOLOaiCAL 




A. GOLD DREDGE WORKING UP THE SWAN FROM VALDORO. 

Is north, down the Swan. The long ridge of gravel is the work of this dredge as far as Valdoro, the gioup of 
buildings in the distance. Below Valdoro the mateiial was stacked by another dredge, which worked down to the 
I, dilapidated machine in front of the larger craft is an old steam dredge that never moved far from 
t was launched. 



lings in tl 

. The s 




thwesl. partly down tl 



e to the right and the Tenm 



GOLD PLACERS. 181 

is advancing up the Swan and the other (PL XXTV, -4, p. 106), after working down the Swan 
from Valdoro, is moving up the Blue. These are electrically driven, the open-connected lines of 
forty-two 9i-foot buckets being actuated by 200-horsepower motors. The total horsepower 
of each dredge is about 450. The dredge on the Blue is designed to dig about 50 feet below 
water and the one on the Swan 40 feet. Both stack from 40 to 50 feet. The daily capacity 
varies from 3,000 to 4,000 cubic yards, depending on the character of the ground. 

In French Gulch are the dredges of the Reliance Gold Dredging Co. and the French Gulch 
Dredging Co. The Reliance dredge is a double-lift machine of special design and is electrically 
driven, with a total horsepower of about 425. This dredge, originally driven by steam, has 
undergone many alterations and in 1909 its open-connected line of forty- two 9-foot buckets 
was replaced by close-connected 5-foot buckets. The actual capacity of this machine in August, 
1909, was at the rate of about 2,800 cubic yards in 24 hours. The material from the buckets, 
after the removal of the bowlders by grizzly and trommel, falls to the bottom of the boat and is 
lifted by a centrifugal pump, requiring 100 horsepower for its operation, before passing over 
the sluices. It is claimed that this second lift scours the gold and renders it more amenable to 
amalgamation and that very large nuggets are occasionally caught on the Reliance dredge that 
would go out over the stacker on the other boats. 

The dredge of the French Gulch Dredging Co., locally known as the Reiling dredge, is an 
electric dredge of the Bucyrus type with a capacity of about 2,000 yards a day. 

The dredges of the Colorado Gold Dredging Co. and the Reiling dredge are supplied with 
power by the Central Colorado Power Co., which generates electricity at Shoshone, on Eagle 
River, about 8 miles from Glenwood Springs. The power for the Reliance dredge is furnished 
by Spruce Creek, south of Breckenridge. The water is delivered at the powerhouse on the Blue 
under a head of 500 feet, and drives dynamos that develop from 500 to 600 horsepower. 

The cost of dredging probably varies considerably, being least on the Swan, where break- 
ages and wear from large bowlders and h%rd bedrock are less than on the Blue or in French 
Gulch. According to Bradford and Curtis,* whose excellent article should be consulted by the 
reader who desires further details regarding the equipment of the Breckenridge dredges, the 
Colorado Gold Dredging Co. in 1908 dredged at an average cost of 8 cents a yard. Certainly 
the cost with a well-constructed dredge on French Gulch ought not to exceed this. On the other 
hand, the figure is probably high for the lower Swan, where the cost is reliably stated to be 
from 4 to 5 cents a yard. 

It is entirely practicable, as Mr. B. S. Revett has shown with the Rehance dredge, to 
operate through the winter, but it is an open question whether this is profitable. The Reliance 
dredge ran through the winter of 1908-9 but lost valuable months in the summer in overhauling 
and alterations. 



1 Bradford, A. H., and Curtis, R. P., Dredging at Breckenridge, Colo.: Mln. and 8cl. Press, Sept. U, 1909, p. 366. 



CHAPTER XV. 

THE GEOLOGIC PAST AND THE ECONOMIC FUTURE OF THE BRECKENRIDGE 

DISTRICT. 

SUMMARY OP GEOLOGIC HISTORY, 

The geologic history of the district, so far as it is known, has been incorporated into the 
preceding pages. A brief narrative recapitulation of it, however, may help to leave a clear 
impression of the salient events fresh in the mind of the reader. 

During the Paleozoic era the area corresponding to the Breckenridge district was at the 
southwest end of an island or peninsula of pre-Cambrian crystalline rocks. These were being 
worn down by active erosion and their debris helped to build up the thick series of generally 
arenaceous, shallow-water sediments on the subsiding sea bottom to the south and west, par- 
ticularly the '* Weber'' and Maroon formations. Through the long lapse of time represented 
by this sedimentation the pre-Cambrian rocks of the land area were eroded nearly to sea level 
and probably were also slowly sinking in company with the neighboring sea floor. 

In the Triassic period, or possibly as early as the Permian, the sea began to creep north- 
ward over the site of the future Breckenridge, depositing as it advanced the red "Wyoming" 
sediments. By the time the shore had encroached about 2 miles north of the site where the 
town now stands the remaining land in this particualr region evidently stood barely above 
water, and as subsidence or submergence continued the Dakota phase of sedimentation over- 
lapped on the worn pre-Cambrian rocks and covered them with a deposit of light-colored sands 
and gray, in part calcareous muds to a depth of 200 to 300 feet. 

The transgression of the sea may not have been wholly uniform, for the Ju;rassic and Lower 
Cretaceous are not certainly represented in the Breckenridge stratigraphic section, and although 
the beds from the '^Sawatch" quartzite to the Upper Cretaceous shale appear to have been laid 
down in undisturbed succession the possibility of there being an unconformity between the 
'^Wyoming" and the Dakota, and perhaps elsewhere in the series, should not be overlooked. 

The waters in which were deposited the comparatively clean arenaceous sediments of the 
Dakota were succeeded by a shallow muddy sea on whose slowly sinking bed accumulated a 
great thickness of dark, more or less carbonaceous muds with subordinate layers of sand and 
of impure carbonate of lime. The rather scanty faunal remains preserved in these silts show 
that they range through the Benton and Niobrara divisions of the Upper Cretaceous and per- 
haps continue up into the Montana. This predominantly shaly deposit accumulated to a thick- 
ness of over 3,000 feet and was doubtless covered in turn by other beds of which no local record 
remains, as they have been removed by erosion. 

The great diastrophic movements at the close of the Cretaceous period put an end to marine 
sedimentation in the Cordilleran region. It was probably during this revolutionary epoch that 
the porphyries were intruded, the beds folded and tilted, and the principal lines of dislocation 
initiated. 

Of events in Tertiary time there is no detailed record. During the early Tertiary, closely 
following the intrusions but, according to Emmons, antedating the major faulting, the lean 
precursors of the present ore deposits were formed. Throughout the rest of the period the 
region was probably subjected to vigorous erosion, in the course of which much concentration 
of the metals in the upper parts of the veins was effected by downward-moving atmospheric 
waters, the major features of the present topography were carved, and considerable gold from 
the veins accumulated in the gravels along the larger streams. 

182 



GEOLOGIC PAST AND ECONOMIC FUTURE OF BRECKENRIDGE mSTRICT. 188 

Of the first advance of the ice in the Quaternary period no direct record remains in the 
vicinity of Breckenridge, but the extensive deposits of coarse gravel, the terrace gravels, which 
formerly clogged the comparatively broad valley of the Blue, are explained as a valley train 
deposited in front of the ice of this earlier stage of glaciation. Whatever morainal material was 
left by the first glaciation has been eroded away or was covered by the deposits of the second 
advance. There must have been considerable erosion between the two glacial maxima, for the 
moraines and valley trains of the second epoch occupy valleys which in most places had been 
cut through the older gravels. The principal streams of the interglacial stage appear indeed 
not only to have cut down to bedrock but to have made some progress in deepening their rocky 
channels before they were again interrupted and overladen with detritus by the second epoch 
of glaciation. 

Since the last retreat of the ice the topography has not been greatly modified. The streams 
have gashed the terminal moraines and have incised narrow ribbon-like flood plains in the low- 
level gravels of the younger valley train. 

FUTURE OF THE DISTRICT. 

It can not be truly said that the Breckenridge district gives definite promise of any general 
and permanent revival of deep mining. In fact, at no time in the history of the district has 
ore been taken from workings such as would elsewhere pass for deep, and at no time has the 
scale of mining operations been really large. The hope, sometimes expressed and more than 
once acted on in the past, that the same limestones so enormously productive at Leadville may 
be found somewhere in the district underneath the younger rocks is dispelled by the demon- 
strable disappearance of the Paleozoic formations northward and the overlap of the Dakota 
on the pre-Cambrian. Although excellent ore has been taken from replacement deposits in 
Gibson and Shock hills the beds containing these deposits are thin and the ore apparently is low 
in grade below the zone of oxidation. The known deposits have never been of a kind to justify 
mining on an extensive scale, and even should new deposits of this character be found it is not 
likely that they would yield ore in large quantities. There are no calcareous beds in the dis- 
trict of sufficient thickness and extent for the development of large deposits by metasomatic 
replacement, 

The Famcomb Hill veins have been virtually exhausted. Although lessees will probably 
find small pockets occasionally, this locality holds out no encouragement for extensive work. At 
their best the veins were exploitable only on a small scale and they are so narrow that the pres- 
ence of extraordinarily rich ore alone can make them profitable. There is a possibility that 
similar veins may be found elsewhere in the shale near some body of porphyry, but placer opera- 
tions have nowhere else indicated such remarkable convergence of rich detrital deposits toward 
one locaUty as has occurred at Famcomb Hill. 

Of the veins belonging to the zinc-lead-silver-gold series a considerable number have proved 
minable only in their upper enriched portions. Some of these may perhaps in future be reopened 
and made productive by improved meth&ds of mining and milling, but this is problematic. 
Another unfortunate feature of these deposits is that they may contain excellent ore in one of 
the flat-lying masses of monzonite porphyry but be wortKless in ^he underlying shale or quartz- 
ite. In other words, the dependence of ore on kind of countiy rock, taken in connection with 
the many lithologic changes generally revealed by deep vertical exploration, is an adverse fac^ 
tor to be reckoned with in addition to the depreciation of the ores below a depth of a few hun- 
dred feet. This feature also enters fully as much into the evaluation of the deposits of the 
gold-silver-lead series as of the lodes exemplified by the Mineral Hill group. 

Against these general disadvantages may be set off some more hopeful considerations. A 
few of the zinc-lead-silver-gold veins, especially those in the Wellington mine, undoubtedly 
contain a large quantity of good ore within a distance of 400 feet from the surface. The prob- 
lem of working successfully the low-grade deposits of the Jessie and Cashier type does not appear 
to have been adequately grasped, past efforts having been directed mainly to the finding and 
stoping of relatively rich streaks, the ore from which was apparently milled by rather crude 



184 GEOLOGY AND OBE DEPOSITS OF BRECKENRIDGE DISTBICT, COLORADO. 

methods. Finally, in a district where concentration of the metals has taken place at so many 
points as at Breckenridge it is very imlikely that all the deposits are known or that there is no 
longer any opening for the prospector. It is safe to predict that discovery of additional ore 
bodies will at some time infuse new life into the mining industry of this district. 

In the districts east of Breckenridge, especially in the Silver Plume and Empire districts, 
there are important veins in the pre-Cambrian rocks. This fact, as well as the occurrence of the 
Laurium vein and of the gold-bearing veins at the head of the Blue, suggests the possibihty 
that some of the known veins of the Breckenridge district may carry ore in the ancient crystalline 
rocks, far below their now visible portions. The indications are, however, that the veins in the 
pre-Cambrian would be smaller than in the rocks above and, having been unaffected by enrich- 
ment, would be of low grade. An attempt to follow a vein from the sedimentary rocks and 
porphyries through a lean zone down into the pre-Cambrian in the hope of finding ore would 
probably be disappointing. 

The rich gulch washings and the most auriferous spots in the terrace gravels have been prac- 
tically worked out, but there still remains much low-grade bench gravel that may at some time 
be washed or dredged, although few data are available regarding the tenor of this material. 
Dredging of the deep channels of interglacial, and perhaps in part of preglacial time has of late 
years become an important and profitable industry. How long it will last, even at the present 
rate of progress with three or four dredges no one can say ; but the available gravels are evidently 
exhaustible and an optimistic estimate of the life of the industry, with four boats in steady opera- 
tion, could hardly concede it more than 10 years, or 15 years at the most. More powerful 
dredges than those in use might, by lowering costs, increase the available reserves of gravel, 
although of course this would be partly offset, so far as the life of the industry is concerned, by 
an increase in the rate of exhaustion. 



INDEX. 



A. Page. 

Acknowledgments IS 

Allanite, occurrence of 90 

Alluvial and detrital deposits, recent, description of 79-80 

Amphibole, occurrence of 89 

Andesine, occurrence of 89 

Apatite, occurrence of 90 

Arctic mine, description of 169-160 

Augite, occurrence of 89 

Azurl te, occurrence of 88 

B. 

Ball, Sydney H., on geology of region east of Breckenridge di»- 

trict.... 15.28 

Barite, occurrence of 91 

Bench placers, description of 176-177 

Bevan district, mines included in 13 

Biotite, occurrence of 90 

Bismuthinite, occurrence of 88 

Blue River, mines of 107 

terrace gravels ea«t of, plate showing 68 

terrace gravels west of , plate showing 70 

upper valley of , view of 76 

valley of, view of 70 

view north of 68 

Bradford, A. H,, on dredging at Breckenridge 24 

Breckenridge district, field work in 18 

future of .. 183-184 

general structure of 63.66-<S7 

deUilsof 67-71 

figure shovring 68,64 

geologic history of, summary of ^ 182-188 

geologic map of In pocket 

geologic sections of, plate showing 66 

literature on 21-24 

mineralogy of 81-92 

mines of 103-107 

mining in, history of ' 16 

region adjacent to, map showing 16 

situationof 18 

topography of .* 16 

map showing In pocket. 

C. 

Calcite, occurrence of 87 

California district, mine included in 13 

Capps, Stephen R., jr.. on glacial deposits of Leadville quad- 
rangle 23,24 

Carboniferous formations. See " Weber; " Maroon. 

Cashier mine, description of 148-150 

general plan of, figure showing 149 

Cerusite, occurrenceof 88 

Chalcopyrite, occurrence of 84 

Channels, auriferous, plate showing 178 

Chlori te, occurrence of 90 

ChrysocoUa, occurrence of 90 

Claims, mining, index to 112-123 

map showing In pocket. 

Colorado, silver, lead, and gold regions in, map showing 14 

Contact deposits, nature of 160-161 

Copper, production of, in Breckenridge district 21 

Country Boy mine, description of 134 

geologic relations in 186 

figure showing 135 

veinsin 136 



Fftge. 

Cretaceous rocks, correlation of 40-42 

description of 86-89 

distribution of 84-36,88 

thickness of 89^ 

See aUo Dakota; Upper Cretaceous. 

Cross, Whitman, on porphyries of Breckenridge district 22 

Curtis, R. P., on dredging at Breckenridge 21 

D. 

Dakota overlap, description of 66-67 

Dakota sandstone, distribution of 34-36 

lithologyof 36-87 

Diopside, occ urrence of 89 

Diorite porphyry, altered, chemical analyses of 96, 97 

altered, diagrams showing 97,98 

mineralogical changes in 99-100 

solutions that produced lOO 

Dredges, plates show ing 106, 180 

Dredging, A. H. Bradford on 24 

in gold placers 180-181 

R P Cnrtison 24 

E. 

Emmons, S. F., on geology and mining in Leadville 22, 23, 26 

on geology and ore deposits of Tenmile district, west of 

Breckenridge 22-23 

Epidote, occurrence of 90 

Extension mine, description of 142 

F. 
FamoombHill, mines of 104-106 

view of 128 

Finch, John W., acknowledgments to 18 

Fissures, general character of 110-111 

Fossils in Upper Cretaceous shale * 41-42 

French Creek tunnel, description of 188 

geologic relations of , figureshowing 138 

stratigraphy of 71 

French Gulch, mines of 103-104 

plate showing 20,106 

Q. 
Galena, occurrenceof 84 

Garnet, occurrenceof 89 

Garrey. George H., on economic geology of region east of Breck* 

enridge district 28 

Germania tunnel, plan of, figure showing 162 

Gibson Hill, blanket ore deposits of 160-161 

mines of 106 

Girty, G. H., on Carboniferous formations In Colorado 26, 27 

Glacial lake beds, description of 79 

Gold, early mining of 16-20 

native, plates showing 80,82,84 

occurrence of 81-83 

production of 20-21 

Gold Dust mine, description of 138-140 

geologic plan of, figure showing 139 

ore body in, figure showing section of 140 

Gold placers, classesof 176 

descriptions of 176-181 

dredging in 180-181 

drilllngin 180 

Gravels, low-level, description of 78-79 

low-level, plate showing 78 

placer, material of 178 

tenor of 179 

Gulch washings, description of 175-176 

Gypsum, occurenceof 91 

185 



186 



INDEX. 



H. Page. 

Hamilton mine, description of 147-148 

geologic relations of, figure showing 148 

Helen mine, description of 136-137 

geologic relations of, figure showing 136 

Hematite, occurrence of 80 

Hillside wash, description of 74-76 

Hooeier Pass, stratigraphic rocks of 27-20 

location of I'* 

Hypersthene, occurrence of f>9 

I. 

Illinois Gulch, mines of 10t>-107 

Irving, J. D., on geology and mining in Leadville 23 

I. X. L. mine, description of I'lO 

J. 

Jessie mine, description of 144-147 

fijwuring in, figure showing 147 

geologic relations of 146 

figure showing 146 

ore bodies of 146-147 

figure showing 146 

ore from, plate showing 126 

view of 144 

Jumbo mine, description of 141-142 

K. 
Kaolinite, occurrenceof 90 

L. 

Labradorite, occurrenceof 89 

Lakes, Arthur, on geology of Summit County, Ck>lo 23 

Lauriummine, description of 168-159 

geologic relations of, figure showing 159 

Lead, production of 21 

Lead-zinc ores, oxidation of 168 

sulphide enrichment of 168-169 

Lefflngwell, K. D. K., on geology of Sawatch Range 23 

limonite, occurrence of 86 

Ling mine, mention of 160 

Little Corporal mine, description of 142 

Little Bailie Barber mine, description of 137-138 

M. 

Magnetite, occurrence of 86 

Malachite, occurrenceof 88 

Marvlne, A. R., on'geology of Breckenridge district 21-22 

MetamorpUsm 98-102 

Metamorphosed sedimentary rocks, photomicrographs of 92 

Metasomatism, description of 94 

Microcline, occurrence of 89 

Minerals, occurrence of 81-91 

ore-forming, paragenesis of 164-165 

Mines of the northern and central parts of Breckenridge dis- 
trict 106-106 

of the southern iMirt of Breckenridge district 106-107 

■hipping ore in 1909 103 

Mining claims, alphabetic list of 112-117 

Identification list of 117-123 

map showing In pocket 

Mining in Breckenridge district, history of 16 

Minnesota district, mine included In 13 

Monzonite porphyry, character of 60-63 

chemical composition and classification of 63-66 

plateshowing 60 

photomicrographs of, plate showing 52 

texture of, figure showing 51 

See also Quartz monzonite porphyr>'. 

Moraines, description of 76-78 

Mosquito fault, descri ption of 64 

Mount Gayot, plate showing 20 

Muscovite, occurrence of 90 

O. 

Ore from Wellington mine, plate showing 124 

from Sallie Barber and Jessie mines, plate showing 126 

from Senator mine, plate showing 160 

rich pocket of, figure showing occurrenco of 163 

Ores, composition of 108 

cost of milling and marketing of 109-110 



Page. 

Ores, deposition of, effects of various wall rocks on 166-166 

deposits of. age of 174 

genesis of 171-174 

gold-silver, in the Dakota quartzite, deposits of 161-162 

lead-zinc, oxidation of 168 

sulphide enrichment of 168-169 

of Farmcomb Hill, oxidation and enrichment of 169-170 

prices paid for 109-110 

silver-lead, blanket deposits of, description of 160-161 

treatment of 109 

vertical variations in 166-167 

Orthoclase, occurrence of 88-89 

Ouray mine, description of 188-140 

geologic plan of, figure Hhowing 139 

Outline of report 9 

P. 

Patton, Horace B., on geology and ore deposits of Montezuma 

district 23-24 

Peale, A, C, on geol(^y of regions south and east of Brecken- 
ridge district 21 

Placers, bench, description of 176-177 

deep, channels containing, plate showing 178 

description of 177-178 

Gold Run, view of 74 

Pollock, W. P., on mining in Breckenridge district 17 

Porphyries, alteration of 96-100 

analyses of 62 

of near-by districts 48 

photomicrographs of , plate showing 46,64 

relations of, to one another 60-62 

relative ages of 71 

types of 44 

variation of molecular constituents of, figure showing 61 

Porphyry, exceptional faciesof 66 

intrusions, metamorphism connected with 93-d4 

of Mount Guyot, description of 67-69 

Pre-Cambrian rocks, distribution of 25 

varieties of 26 

Propylltization in monzonite porphyry 101-102 

Puzzle mine, distribution of 138-140 

geologic plan of , figure showing 139 

Pyrite, occurrence of 86 

replacement of Bhale-by, figureshowlng 85 

Q. 
Quartz, occurrence of 85 

Quartz monzonite porphyry, alteration of 100 

character of 44-45 

chemical changes in 101 

chemical composition and classification of 46-60 

intermediate type of, character of 67-68 

chemical composition and classification of 68-69 

distribution of 57 

plateshowing 68 

textureof, figureshowlng 58 

various fades of 6^-60 

mineralogical changes in 101 

ores in, plate showing 126 

propylltization of 101-102 

silicic varieties, plate showing 44 

sill of, figureshowlng 66 

texture of, figureshowlng 44 

See aUo Monzonite porphyry. 

Quartzite, Dakota, etched surface of, plate showing 160 

Quaternary deposits, alluvial and detrital deposits In 79-80 

glacial lake beds in 79 

low-level gravels In 78-79 

moraines in 76-78 

older hillside wash of 74-76 

terrace gravels of 72-74 

R. 

Raymond, B. W., on history of Breckenridge district 21 

Replacement of shale by pyrite, figure showing 85 

8. 

Sallie Barber mine, description of 137 

ore from, plate showing 126 



INDEX. 



187 



Page. 

Seminole open stope, Tlew of 144 

Senator mine, description of 169 

ore from, plate showing 160 

plan of, figure showing 160 

Sericite, occurrence of 90 

Shock Hill, blanket ore deposit! of 160-1 61 

Siderite, occurrence of 87 

plate showing 124 

Silver, early mining of 16-20 

occurrence of 83 

production of 21 

Smithflonite, occurrence of 88 

plate showing 126 

Solutions, ore-bearing, action of 172-174 

South Elkhorn tunnel, plan of 162-163 

plan of, figure showing 162 

Sphalerite, occurrence of 84 

plate showing 1 24, 126 

Spurr, Joidah £., on economic geology of region east of Breck- 

enridge district 23, 26 

Stratified rocks, succession of, in Breckenridge district 26, 31 

Sulphur, occurrence of 81 

Swan River, mines of 105 

valley of, plate showing 76 

T. 

Tectonic block, Breckenridge, eastern boundary of 64 

Tellurides, absence of 83 

Tenmile Range, plate showing 18,104 

Terrace gravels, character of material in 73-74 

correlation of, with deposits outside the district 74 

distribution and thickness of 72-73 

plates showing 68,70,72,104 

Tilden, G.C., onstratigraphy of Eagle County 26 

Titanite, occurrence of 90 

Triassic. &e" Wyoming." 

U. 

Union dlfltilet, mines included in 18 

Upper Cretaceous shale, correlation of : 40-42 

distribution of 88 

lithologyof 88-89 

thicknenof 89-10 



V. Page. 

Van Horn, Frank R., on proustite and argentite near Monte- 
zuma, Colo 23 

Veins, classification and distribution of 11 1-112 

gold, Farncomb Hill type of, character of 166-157 

Famcomb Hill type of, character of, figure showing. . . 166 

country rock surrounding 165 

distribution and geologic relations of 153-16 1 

nature of fissuring shown by 156 

figure showing 154 

oxidation and enrichment in 169-170 

underground workings of 154-165 

in the pre-Cambrian rocks, dcbcriptlon of 158 

examples of 158-lGO 

gold-silver-lead series of, description of 143-144 

strike and dip of 110-111 

zinc-lead-silver-gold, series of 124-142 

description of 124-125 

examples of 126-142 

material of 125 

W. 

Wapiti group, plan of 154 

Ward, Dr. W S.. acknowledgments to 13 

Washington mine, description of 141 

Water level, position of 167-168 

Wellington mine, fauUing in 131-133 

geologi c relations in 127-1 28 

figure showing 129 

history of 126 

oxidation of ore in 134 

plan of 180 

veins in 128-181,188-184 

view of 128 

Westgate, Lewis O., on glacial geology of Leadvllle region 23 

Wire Patch mine, description of 150-152 

geologic relations of , figure showing 151 

"Wyoming" formation, correlation of 83-34 

distribution of 81 

Uthologyof 81-88 

Z. 

Zinc, production of 21 

2Urcon, occurrence of 89 



O 



Ifr^t/n^njiA^^ 



PROFESSIONAL PAPER 75 PUTE II 



SLD 



I 




DEPARTMENT OF THE INTERIOR 
UNITED STATES GEOLOGICAL SURVEY 

QBOKOE OTIB BHTTH, DniCTQB 

Fbofbssionaz. Paper t6 



THE 



SAN FRANCISCAN VOLCANIC FIELD 



ARIZONA 



HENRY HOLLISTER ROBINSON 




WASHINGTON 

GOVBBNHENT PBINTINQ OFFICE 
1918 



CONTENTS. 



iNTBODUOnON , 

Location of area 

Outline of the report. . 
Extent of field work. . 
Previous presentation. 
Acknowledgments 



9 
9 
9 
9 
10 
11 



Previous geologic work 11 

Chapter I . Geography 13 

Physiographic divisions of Arizona 13 

Geography of the volcanic field 15 

Topography 15 

Drainage 16 

General characteristics 16 

Special characteristics 17 

Climate 18 

Forests '. 19 

Chapter II. R^ional geolc^^y 20 

Topics covered 20 

Outline of geologic history based on the sedimentary formations 20 

Sedimentary rocks 20 

Occurrence and general section 20 

Paleoasoic formations 21 

Redwall limestone , 21 

Aubrey group 22 

Subdivisions and nomenclature. 22 

Supai formation (**Lower Aubrey " red sandstone and shale) 22 

Coconino ("Upper Aubrey ") sandstone 23 

Kaibab ("Upper Aubrey ") limestone 24 

Comparison of Pennsylvanian formations in southern Plateau country 25 

Triassic and Permian (?) rocks 26 

Shinarump group 26 

Comparison and correlation of Triassic and Permian (?) formations 28 

Origin of Triassic and Permian (?) formations 28 

Post-Pennsylvanian land area of southwestern Arizona 30 

Glaciation and alluviation of San Francisco Mountain 31 

Geologic structure 33 

General character 33 

Folds 33 

Faults 1 36 

Chapter III. Geology of the volcanoes and lava fields 38 

Outline of chief events 38 

First general period of eruption 38 

Second general period of eruption 40 

Volcanoes of the second period 40 

San Francisco Mountain 40 

Topography 40 

General structure of cone 42 

Eruptive history 42 

Varieties of lava 42 

First stage of eruption 43 

Second stage of eruption 44 

Third stage of eruption 46 

Fourth stage of eruption 47 

Fifth stage of eruption 48 

Volume of the cone and of the individual lavas 49 

Proportion of cone eroded since cessation of volcanic activity 50 

Platform of the volcano 51 

Age of the volcano : 52 

Summary 52 

3 



4 COKTENTS. 

Chaftbr III. Geology of the volcanoes and lava fields — Continued. 
Second general period of eruption — Continued. 

Volcanoes of the second period — Continued. page. 

Kendrick Peak 53 

Topography 53 

Eruptive history 54 

Varieties of lava 54 

First stage of eruption 55 

Second stage of eruption 56 

Third stage of eruption 57 

Fourth stage of eruption 57 

Volume of the cone and of the individual lavas 57 

Proportion of cone eroded since cessation of eruptions 58 

Age of the cone 58 

Summary 58 

Bill Williams Mountain 59 

Topography 59 

Eruptive history 60 

Varieties of lava 60 

First stage of eruption 61 

Second stage of eruption 61 

Volume of the cone and of the lavas of the two stages 62 

Proportion of cone eroded since cessation of volcanic activity 63 

Age of the cone 63 

Summary 63 

O'Leary Peak 68 

Topography 63 

Eruptive history 64 

Varieties of lava 64 

First stage of eruption 64 

Second stage of eruption 65 

Volume of the cone and of the two lavas 65 

Summary 65 

Sitgreaves Peak 66 

Topography 66 

Structure of the main cone 66 

Eruptive history 66 

Volume of the cone 67 

Proportion of the cone eroded since cessation of volcanic activity 67 

Mormon Mountain 68 

Topography 68 

Eruptive history 68 

Volume of the cone 68 

Observatory Mesa 69 

Sugarloaf Hill 69 

Dry Lake Hills 70 

Laccoliths of the second period 70 

Marble Hill 70 

Elden Mountain , 74 

Situation and topography , 74 

Igneous rock 74 

Sedimentary rocks 76 

Eastern sedimentary area 76 

Northern sedimentary area 77 

Origin of Elden Mountain 78 

Mode of formation 81 

Age of Elden Mountain 84 

Summary 84 

Slate Mountain 86 

Summary of second general period of eruption 86 

Third general period of eruption 87 

Chaftbr IV. Geologic history of the volcanic field and adjacent country 91 

Correlation of events 91 

First general period of volcanic activity 91 . 



CONTENTS. 5 

Ghaptbr IV. Geologic history of the volcanic field and adjacent country— Continued. page. 

Uplift and erosion interval following first period of eruption 92 

Second general period of volcanic activity 93 

Uplift and erosion following second period of eruption 93 

Third general period of volcanic activity 93 

Chaftbr V. Petrography 96 

Method of treatment 96 

Reference types 97 

Classification 97 

Types 99 

Granite-rhyolite 99 

Syenite-trachyte ' 99 

Diorite-andesite 100 

Gabbro-hasalt 101 

Granite-rhyolite-syenite-trachyte 101 

Granite-rhyolite-diorite-andesite 102 

Syenite-trachyte-diorite-andesite 102 

IMorite-andesite-gabbro-basalt 103 

The rhyolitic lavas 108 

Occurrence and general character 108 

No . 1 . Biotite rhyolite 103 

No. 2. Biotite-soda granite porphyry 106 

No. 3. Biotite-soda rhyolite 107 

No. 4. Riebeckite-soda rhyolite 109 

No. 5. Riebeckite-soda granite porph3rry 110 

The dadtic lavas 114 

Occurrence and general character 114 

No. 6. Biotite dacite of Kendrick Peak 116 

No. 7. Biotite dacite of O'Leary Peak 117 

No. 8. Biotite-homblende dacite 118 

No. 9. Hypersthene-soda dacite 121 

No. 10. Hypersthene-homblende-soda dacite 124 

No. 11. Hornblende-soda dacite of Mormon Mountain 126 

No. 12. HomblendeHsoda dacite of Bill Williams Mountain 127 

No. 13. Pyroxene dacite 130 

No. 14. Hypersthene dacite 132 

No. 15. Hornblende dacite 134 

The latitic lavas 136 

Occurrence and general character 136 

No. 16. Pyroxene latite 136 

No. 17. Pyroxene-hornblende latite 137 

The andesitic lavas 139 

Occurrence and general character  139 

No. 18. Augite andesite of San Francisco Mountain 140 

No. 19. Augite andesite of Kendrick Peak 143 

No. 20. Augite andesite-basalt 145 

No. 21. Hornblende-soda andesite-basalt 146 

The basaltic lavas 149 

No. 22. Augite basalt of Cedar Ranch Mesa 149 

No. 23. Augite basalt of Kendrick Peak 161 

Chapter VI. Petrology 165 

Outline of discussion 155 

Composition of the average lavas of the three general periods of eruption 155 

First period 156 

Second period '156 

San Francisco Mountain 156 

Kendrick Peak .* 157 

Bill Williams Mountain 158 

O'Leary Peak 158 

Mormon Mountain 158 

Other localities 159 

Average lava of second period 159 

Third period 160 



6 CONTENTS. 

GKAFrERVI. Petrology — Continued. Pago. 

Average lava of the San Franciscan volcanic field 160 

Method of determination 160 

Chemical composition 161 

Mineral composition 161 

Homogeneity of the average lavas of the composite cones 162 

Average composition of the main rock types 163 

Rhyolite 164 

Dacite 164 

Latite 164 

Andesite 165 

Basalt : 165 

Differentiation of the lavas in space 165 

Period of occurrence 165 

The rhyolites 166 

Thedacites 167 

Thelatites 167 

The andesites 168 

Summary and conclusions 168 

Differentiation of the lavas in time 169 

San Francisco Mountain 169 

KendiickPeak 170 

O'Leary Peak • 171 

Bill Williams Mountain 171 

Mormon Mountain 172 

Summary and conclusions 172 

Differentiation of the magma 173 

• Zone of differentiation 173 

Relative proportion of the erupted lavas to the total volume of the magma 174 

Degree of similarity between the erupted lavas and their magmas 175 

Schemes of differentiation 176 

First scheme 176 

Second scheme 176 

Conclusion * 177 

Chemical characters 177 

Chemical analyses 177 

Molecular ratios of the principal oxides 179 

Absolute chemical characters 179 

Serial chemical characters 180 

Method of representation 180 

Changes in the individual oxides 182 

Grouping of oxides 184 

Extrapolation of the oxide curves 184 

Conclusions 185 

Ratio of alumina to lime and alkalies 185 

Ratio of the alkalies 186 

Summary of the chenucal characters of the lavas 188 

Mineral characters 188 

Outline of discussion 188 

General characters 190 

Serial mineral characters 194 

Method of representation 194 

Changes in individual minerals 195 

Discussion of results 197 

Division into groups 197 

Group 1 197 

• Group 2 198 

Summary 200 

Conclusions 200 

Specific gravities 203 

Texture 206 

Crystallinity 207 

Granularity 209 

Index 211 



ILLUSTRATIONS. 



Page. 

Plate I. ^, Black Bill Park, east of San Fmuciijco and Elden moiinUine; B, Oak Creek Canyon 16 

II. ^, Coconino monocline at Coconino Point, Little Colorado Valley; i?, Bottomless Pit> south of Elden 

Mountain 17 

III. General geologic map of the San Franciscan volcanic field, Arizona 20 

IV. A, Cherty Kaibab ("Upper Aubrey") limestone at cliff dwellings in Walnut Canyon, southeast of 

Flagstaff; B, Cross-bedded Coconino ( ** Upper Aubrey " ) sandstone in Walnut Canyon 22 

V. Geologic map of San Francisco Mountain and vicinity 40 

VI, Geologic cross sections of San Francisco Mountain and vicinity 40 

VII. A, San Francisco Mountain from Valley Bouito on the east; B, San Francisco Mountain from Crater 

Hill on the south 42 

VIII. A, Inner (southeast) slope of San Francisco Peak from Core Ridge; B, Neck of augite andesite of fifth 

stage of activity 43 

IX. Ay Kendhck Peak from the south; B, Bill Williams Moimtain from the west; C, Sitgreaves Peak from 

the west 54 

X. A, Elden Mountain from Observatory Mesa on the west; B, Structure and texture of the igneous rock 

of Elden Mountain near Doyle's spring, at northwest base 76 

XI. Basalt cones, belonging to third period of eruption: A^ On northeast edge of field; B, On southeast 

edge of field 88 

XII. A, Dissected ash cone of Red Moimtain at head of Hull Wash; B^ Sunset Peak; C, Very recent conelet 

near Sunset Peak 89 

XIII. Granite porphyry of San Francisco Mountain: A^ Variety containing segregated areas; B, Normal 

variety; C, Abnormal variety 110 

XIV. Photomicrographs of typical lava textures: A^ Soda rhyolite; B, Soda dacite; C, Latite; D, Andesite; 

E, Basalt of first period of eruption; F, Basalt of third period of eruption 206 

Figure 1. Index map showing location of the San Franciscan volcanic field, Arizona 10 

2. Map showing junction of Colorado and Little Colorado rivers as located by Ives and Newberry. ... 12 

3. Sketch map of Grand Falls, Little Colorado River 17 

4. Profiles of Mount Shasta and San Francisco Mountain 41 

5. Sketch showing location of main vents and directions of fiows, San Francisco Mountain 43 

6. Section through Peak A and east slope, San Francisco Mountain 44 

7. Section through San Francisco Peak, Peak A, and Schulz Peak, San Francisco Mountain 45 

8. Comparative erosion of San Francisco Mountain and Mount Taylor 51 

9. Topographic map of Kendrick Peak 53 

10. Geologic cross sections of Kendrick Peak 54 

11. Topographic map of Bill Williams Mountain 59 

12. Geologic cross sections of Bill Williams Mountain 60 

13. Profiles of Sitgreaves Peak 66 

14. Plan of Sitgreaves Peak and adjacent cones 67 

15. Cross section of valley half a mile north of Doyle's spring 70 

16. Topographic and preliminary structure map of Marble Hill 71 

17. Geologic cross section through Marble Hill 72 

18. Geologic map of Elden Mountain 75 

19. Cross section from Switzer Mesa to west slope of Elden Mountain 78 

20. Cross section through north slope of Elden Mountain 79 

21. Croas section through east slope of Elden Mountain 79 

22. Plan of Elden Mountain 81 

23. Geologic cross sections of Elden Mountain 83 

24. Map of San Franciscan volcanic field showing distribution of cones 88 

25. Relative positions of cones of second period with respect to San Francisco Mountain 166 

26. Hypothetical form of magmatic reservoir 169 

7 



8 ILLUSTRATIONS. 

Pace. 

FioiTBB 27. Schemes of di£ferentiation of the magma in the San Fianciacan volcanic field 176 

28. Serial chemical characters of the lavas of the San Franciscan volcanic field 181 

29. Ratio of alumina to lime and alkalies 186 

30. Ratio of alkalies 187 

31. Ratio of alumina to lime and alkalies in the feldspare 192 

32. Serial mineral characters of lavas of the San Franciscan volcanic field 194 

33. Relation of mineral groups 1 and 2 in the lavas of the San Franciscan field 197 

34. Distribution of analyses of igneous rock in general with respect to silica 202 

35. Specific gravities of the lavas of the San Franciscan volcanic field 204 

36. Relation between amount of groundmaas and chemical composition of the lavas 208 



THE SAN FRANCISCAN YOLCANIC FIELD, ARIZONA. 



By Henby Hollister Robinson. 



INTRODUCTION. 

LOCATION OF AREL^. 

The San Franciscan volcanic field, which takes its name from San Francisco Mountain, 
the largest volcano of the group, covers about 3,000 square miles in the north-central part of 
Arizona, as shown by the shaded space on the index map forming figure 1 . The center of the 
field lies about 50 miles south of the Orand Canyon of the Colorado and the southern boundary 
is in part coterminous with that of the San Francisco Plateau, which forms the southwestern 
division of the great Colorado Plateau. 

The region is easily reached, for the main line of the Atchison, Topeka & Santa Fe Railway 
traverses it from east to west for more than 60 miles. FlagstaflP, a town of 1,500 inhabitants 
10 miles south of the summit of San Francisco Mountain, is on the railroad, and a branch line 
runs from Williams, 34 miles farther west, to the Orand Canyon. All the more important 
points of interest in the field may be reached without difficulty by wagon, and outfits may be 
obtained at Flagstaff. 

OUTLINE OF THE REPORT. 

This report deals primarily with the volcanic phenomena of the region as determined in 
the field and laboratory. Chapter I contains a brief description of the geography of the field 
and Chapter II is devoted largely to the sedimentary formations and structure. The rest of 
the report — Chapters III to VI — treats entirely of tie various features of the volcanoes and 
igneous rocks, both individually and collectively. Detailed descriptions of the volcanoes 
and lava fields are given in Chapter III; the volcanic history of the region and its correlation 
with the general history of the surrounding country are presented in Chapter IV. These two 
chapters will presumably suffice for the general reader who may desire to become acquainted 
with the broader volcanic features of the region. Chapter V (Petrography) is devoted entirely 
to the detailed description of the individual igneous rocks of the region, as represented by a 
selected set of type specimens. In Chapter VI (Petrology) is presented a discussion of the 
igneous rocks considered collectively — tliat is, as a series of genetically related members. 
These last two chapters will be more especially interesting to petrologists, although there ia 
considerable matter in the last chapter which may also be of interest to the general reader. 

EXTENT OF FIELD WORK. 

The field work on which the report is based was carried on during the summers of 1901 to 
1903, a portion of the time, however, being occupied by side trips to the Grand Canyon of the 
Colorado, the Verde Valley, and the Moqui Buttes. It was the original intention to study 
only San Francisco Mountain, but scattered observations made during the first summer at 
other localities, especially at Elden Mountain and Kendrick Peak, seemed to indicate that the 
region would repay wider study. The work was accordingly extended so as to embrace all 
the large cones that lie in the vicinity of San Francisco Mountain and some 2,000 square miles 
of the surrounding plateau country. The more detailed work was confined to the large cones 

9 



10 



SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 



and the laccoliths, as they presented the greatest variety of phenomena within the smallest 
space. Reconnaissance work was carried on in the surrounding country more especially for 
the purpose of determining the limits of the widespread basalt flows, their relation to the 
underlying sedimentary formations, and the character of those formations. 




FiQUBE 1.— Index map showing location of the San Franciscan volcanic field, Arixona. 

PREVIOUS PRESENTATION. 

In 1903 the results of the first two summers' field work were presented, under the title 
'^Geology of San Francisco Mountain and vicinity, Arizona, '^ as a thesis in partial fulfillment 
of the requirement for the doctorate degree at Yale University, New Haven, Conn. This 



INTBODUCTION. IX 

thesis, however, fonias only a minor part of the present paper, for the third season in the field 
and much additional laboratory work have greatly expanded the scope of the report. 

ACKNOWLEDGMENTS. 

The chemical analyses of the igneous rocks, with two exceptions, were made by the writer 
in the mineralogical laboratory of the Sheffield Scientific School, and he wishes to record his 
indebtedness to the late Prof. S. L. Penfield for many suggestions made throughout the course of 
the analytical work. The writer wishes also to express his thanks to Mr. S. H. Clapp, for the 
analysis of the rhyolite (No. 1) from Sugarloaf Hill and to Mr. R. J. Marsh for that of the 
hornblende andesite-basalt (No. 21) from Bill Williams Mountain, both of which were made 
in duplicate under the direction of Prof. H. W. Foote in the chemical laboratory of the Sheffield 
Scientific School; and to Prof. F. N. Guild, of the University of Arizona, for the analyses of the 
Redwall limestone, the cherty Kaibab limestone, and the residual clay resulting from the decom- 
position of the basalt of the first period of eruption. To Prof. L. V. Pirsson, Prof. Joseph . 
BarreU, and Dr. H. M. Dadourian the writer is greatly indebted for advice on various parts 
of the work. 

PREVIOUS GEOLOGIC WORK. 

The first geologist to visit this region was Jules Marcou in 1853. He was followed by J. S. 
Newbeny in 1857 and G. K. Gilbert in 1872-73. The region was not further studied until 1901, 
when the writer began his work. Since that time, however, several brief studies of certain 
features of the geology have been published, and they are referred to in later pf^es of this report. 
The early geologists accompanied expeditions whose main objects were either the location of 
lines of communication between the East and the Pacific coast or the acquirement of general 
information in regard to the country Their observations were restricted, necessarily, to the 
geology in the inmiediate vicinity of the routes of the expeditions and consequently were not 
always suitable or sufficiently extensive to be used as a basis for generalizations. It will be in- 
teresting, however, to review briefly the work of these geologists more especially as it relates 
to the volcanic phenomena of the region. 

Marcou's observations ' in the San Franciscan region were very brief, as he crossed it in 
midwinter, when the ground was covered with snow. His interest, however, was in the sedi- 
mentary rather than the volcanic rocks. The first three sentences quoted below are especially 
interesting historically and as a reminder of the controversy to which his proposal gave rise. 
He wrote: 

Shortly after quitting the Ghiquito River we found here with the la^t beds of red clay of the Trias, and in con- 
cordant stratification, a magnesian or dolomitic limestone, with very regular strata from half a foot to 1 foot in thick- 
ness. Several beds contain fossils badly preserved, among which I recognized, however, a Nautilus, a Pterocerafi, 
and a BelemniteB. This formation, which is placed between the Carboniferous and Trias, corresponds without doubt 
to the magnesian limestone of England and is a new member which I add to the series of secondary rocks in North 
America. *   From the Sierra of San Francisco to Cactus Pass the geology * * * is very complicated on 
account of the immense extinct volcanoes which have covered with their lavas and basaltic streams the sedimentary and 
granitic rocks that primitively formed this region. * * * There are four or five extinct volcanoes over this space, 
the largest being that of San Francisco, which is 12,000 feet above the level of the sea. 

Newberry says: 

To the casual observer the San Francisco Mountain forms a most impressive feature of the scenery which surrounds 
it, not only from its symmetrical and striking outlines but also from its isolation. * * * Its geological structure 
fully accords with its physical aspects. It is volcanic throughout and is, in fact, a huge volcano whot« fires have been 
but recently extinguished. Through one great and several minor vents, opened in the strata of the high mesa, where 
they have a thickness of at least 5,000 feet, a vast quantity of lava has been poured, covering with a flood of melted 
matter the country for many miles around and forming one principal cone with a thousand inferior ones. * * « 
Little disruption of the stratified rocks attended this grand exhibition of volcanic force, and the formation of the moun- 
tain seems to have been effected entirely by the ejection of matter in a state of complete fusion, through narrow 
orifices of unfathomable depth.' 

1 liaroou, Jules, RdBom^ of a geological reoonnalssance, etc.: Repts. Expl. and Surveys for RaUroad from ICississippi River to Padflo 
Ocean (S. Ex. Doc. No. 78, 33d Cong., 2d sess.), vol. 3, 1856, pt. 4, p. 170. 

• Newberry, J. S.. Report upon the Colorado River of the West, War Dept., 1861, pt. 3, pp. dS-66. 



12 



SAN FRANCISCAN VOLCANIC FIELD, ABIZONA. 



The existence of great volcanoes like San Francisco Mountain and San Mateo [Mount Taylor in New Mexico] in 
full blast many hundred miles from the sea is a powerful argument against the theory which restricts all volcanoes 
to the vicinity of lai^ge bodies of water.* 

In reading the report of the Ives expedition, of which Newberry was a member, the writer 
experienced some difficulty in understanding certain passages. The confusion was caused by 
the incorrect location of the junction of Colorado and Little Colorado rivers from 70 to 85 
miles too far west. Ives and Newberry also differed as to the location of the junction, as may 
be seen from the following quotations and figure 2. This probably accounts for the indefinite 
indication of their map of the course of Colorado River north of its supposed junction with the 
Little Colorado. Ives ' says: 

A trail was encountered * * * [which] headed directly for the north side mountains [Mount Trumbull] — the 
peaks already spoken of as seen on the opposite [north] bank oftite Colorado. * * * A good view was obtained of the 
walls of the Flax [Little Colorado] River canyon and its mouth approximately located. The junction was below the 
mouth of Cascade [Cataract] Creek, showing that that stream is not * * * a tributary of the Colorado. 

Newberry's account,' written from the same locality as that of Ives, says: 

The angle of the mesa included between the Great and Little Colorados, on the north side of the latter streamy 
* * * overlooks the valley in a nearly perpendicular wall [Mount Trumbull] some 4,000 feet in height. * * * In 
the northwest the high mesa at the junction of the two Colorados formed a most conspicuous object. 

Ives thus made the junction east of Mount Trumbull, whereas Newberry placed it on the 
west side. As the result of supposing the junction at this locality, Newberry describes the 



NOITTH SIDE MTS. / 
(Mt.Tfwmbull) ; 




oro> 



<»^ 



Red Butte 



^j 



FiouBB 2.— Map ahowing Junctioa of Colorado and Little Colorado rivers as located by Ives and Newberry. A, Location by Ivea;^, locatkm by 

Newberry; C, correct location. 

region between the present Aubrey Cliffs and Coconmo Plateau as the valley of the Little 
Colorado, whereas it is actually that of Cataract Creek. 

The work of G. K. Gilbert,* although a reconnaissance, was more extended than that of 
Marcou and Newberry. He noted, in adition to the rocks mentioned in the quotation below, 
the presence of a small amount of rhyolite in San Francisco Mountain. He determined that — 

The San Francisco group includes a series of large cones of trachyte, the product of massive eruptions, and a great 
number of small basaltic cones, associated with broad and in part thick sheets of basaltic lava. The trachyte has per- 
haps the greater mass, but the basalt covers by far the greater area. The large cones, though they may be justly called 
a group, are separated by intervab of several miles. 

It is well worthy of note that the majority of these eruptions among the plateaus rest upon nearly level strata, 
* * * where the structure is so simple    [that] a local structure imposed by the extrusion of lava could 
not escape detection, and we *have direct evidence in its absence that the erupted rocks, in passing through, have 
not uplifted the sed^'mentary. This remark applies not merely to the eruptions of basalt, which we know * *  
to have been a tolerably thin fluid, but also to the most viscous trachyte, which, in the case of San Francisco Moun. 
tain, for example, has been built, not a scoriaceous mass, but a pyramid of compact lava, to a height of nearly 5,000 
feet, with slopes of 10° to 20**. 

1 Newberry, J. S., Report of escploring expedition from Santa Fe, N. Mex., to Junction of the Grand and Oreen rivers of the Great Colorado of the 
West, in 1859, War Dept., 1876, p. 62. 

• Ivee, J. C, Report upon ♦he Colorado River of the West, War Dept., 1861, pt. 1, p. 110. 
•Op.clt., pt. 3, p. 61. 

* v. S. Oeog. Surveys W. 100th Mer., vol. 3, pt. 1, 1875, Geology, pp. 129-131. 



CHAPTER L 

GEOGRAPHY. 

PHYSIOGRAPHIC DIVISIONS OF ARIZONA. 

The physiographic divisions of Arizona were first broadly outlined by Gilbert/ who recog- 
nized the Plateau and Basin Range provinces. Later Glassford' further divided the Basin 
Range province into the Plain and the pro-Plateau. A similar threefold division, under the 
headings Plateau, Mountain, and Desert districts, was adopted by Ransome ' in his reports on 
the Globe and Bisbee copper districts. The same divisions are recognized in this report under 
a slightly different and more uniform nomenclature. The region is thus divided into the Plateau 
and Basin Range provinces, and the Basin Range province is subdivided into the Mountain and 
Plains districts. (See fig. l, p. 10.) 

The Plateau and Basin Range provinces are, on the whole, topographically distinct from 
each other. The former is a broad expanse of nearly horizontal strata disturbed in only a few 
places; the latter is a region of faulted and tilted block mountains separated by flat-floored 
valleys. The difference between the Mountain and Plains districts is of another order. Through- 
out both districts there are short isolated mountain ranges from 1,000 to 4,000 feet high, with 
parallel trend. The intervening rock-floored or waste-filled valleys are essentially flat. The 
distinction here is in the comparative abundance of similar topographic forms. In the Moun- 
tain district the areas occupied by the ranges and valleys are about equal in extent; in the 
Plains district the area covered by the valleys largely predominates over that covered by the 
ranges. 

The Plateau province occupies the northeastern portion of Arizona, comprising an area of 
about 45,000 square miles, or 40 per cent of the State. To the north and east it extends into 
the adjacent States. From the Colorado to the headwaters of Salt River it terminates, for the 
most part sharply, in the Grand Wash and Aubrey cliffs, which rise to heights of 1,000 to 2,000 
feet and expose in their faces the horizontal strata of the Plateau. The general elevation of 
the region is 6,500 feet and is much greater than that of the Basin Range province. The 
sunmiits of several of the volcanoes upon its smrface have elevations of over 10,000 feet; only 
along Colorado River and its tributaries does the elevation of the Plateau fall below 5,000 feet. 
The essential horizontality of the strata in the Plateau province, from which there are but 
slight departures except where there has been monoclinal folding, is in marked contrast with the 
structure in the Basin Range province. The fact that the strata are practically level has proba- 
bly given rise to the idea that the surface is smooth, as is implied in the expression '4evel mesa," 
not infrequently applied to it. The contrary is perhaps nearer the truth, for erosion has cut 
many profound canyons, including the Grand Canyon of the Colorado, which attains a depth 
of over 5,000 feet. Volcanic forces have been active at numerous localities and the erupted 
lavas have built up many large and small cones and spread out over thousands of square miles. 
Two opposing forces, therefore, have produced the detailed topography of the Plateau — erosion, 
which is destructive, and volcanism, which is constructive. 

The Basin Range province comprises the entire southwestern part of the State and is larger 
than the Plateau province, for it covers 68,000 square miles. It is divided into the Mountain 
and Plains districts. 



» Gilbert, 0. K., U. 8. Oeog. Surveys W. 100th Mer., vol. 3, 1875. chap. 1. 

> Olassford, Lieut. W. A., Report on the climate of Aritona; Ex. Doc. 287, filst Con^., 2d sees. 

» Ranaome, F. L., Geology of the Globe copper district, Ariiona: Prof. Paper U. S. Geol. Survey No. 12, 1903; Globe folio (No. Ill), Geol. Atlas 
U. S., U. S. Geol. Survey, 1904; Geology and ore deposits of the Bisbee quadrangle: Prof. Paper U. S. Geol. Survey No. 21, 1904; Bisbee folio (No. 
112), Geol. Atlas U. 8., U. S. Geol. Survey, 1904. 

13 



14 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

The Mountain district lies between the Plateau province and the Plains district and is the 
smallest of the three physiographic divisions, covering 27,000 square miles, or 24 per cent of the 
region. It extends southeastward from the Colorado to Salt River as a belt of country about 
50 miles wide. At Salt River it widens out, the western boundary running southward into 
Mexico. Farther east the district coalesces with a similar one in New Mexico and extends a 
short distance beyond the Rio Grande. In this district there are some thirty mountain ranges 
of the basin-range type, which occupy, as estimated from the Geological Survey's large-scale 
topographic map of the United States, 15,000 square miles, or 55 per cent of the area, thus 
leaving 45 per cent for the valleys. The average area covered by a range is 500 square miles 
and the ratio of its width to its length is 1:2.8. The entire region stands at a higher gfeneral 
elevation above sea level than the Plains district, but lower than the Plateau. The mountains 
range in height from about 4,000 to 6,000 feet and rise 2,000 to 4,000 feet above the valley 
bottoms. The summits of the Bradshaw and Pinal mountains, however, are nearly 8,000 feet 
abot-e the sea. Throughout the district the ranges exhibit a marked parallelism in trend, which 
in general is northwest. At the Mexican boundary, however, the trend is nearer north-northwest, 
^nd at Colorado River it swings around to north, A direction in which it contitiues through 
Nevada. 

An estimate of the relative areas of range and valley in the Basin Range country was made 
from the map which accompanies Spurr's paper on the geology of Nevada south of the fortieth 
parallel.^ Exactly the same values were obtained as for the Mountain district of Arizona, and 
although this should be considered a coincidence, it illustrates the intimate relation of the two 
regions. In Nevada, however, there are sixty ranges covering 12,000 square miles, against 
thirty covering 15,000 square miles in the Mountain district of Arizona. The area covered by the 
average range in Nevada is thus much smaller and the ratio of the width to the length of the 
ranges is also less, being 1:2. 

The mountains and valleys in the Basin Range country are so sharply defined that it is 
possible to formulate a quantitative definition of the topography of the region, namely, that 
typical basin-range topography is represented by a region where the area occupied by the mountain 
ranges due to block faulting is equal to thai occupied by the valleys. It will be understood that this 
definition applies primarily to the ranges of the Great Basin, an arid region with interior drainage, 
and to the ranges in their present stage of development. With only slight modification the 
definition may be extended to cover the ranges in Arizona, an arid region with exterior drainage. 
Definitions of this character seem preferable to qualitative definitions, but various difficulties 
would have to be overcome in formulating them for the more complex physiographic types. 
The science of physiography is probably not in the stage of development where too great a 
refinement of definition should be attempted, although the quantitative idea might be bene- 
ficially used in the description of the more simple and clearly marked types, of which basin-range 
topography is an example. 

The Plains district comprises the remaining 41,000 square miles, or 36 per cent, of Arizona. 
It is distinguished from the Mountain district by the presence of wide and long level valleys 
separated by rugged isolated mountain ridges 1,000 to 2,000 feet in height. The valley area 
clearly predominates over that covered by the ridges. An estimate based on the large-scale 
map of the United States shows that the valleys occupy 85 per cent of the area, thus leaving 
15 per cent for the ridges. This is practically the same as McGee's estimate ' that one-fifth 
(20 per cent) of the Sonoran district of Mexico, which lies immediately to the south, consists 
of mountains. The valleys have been described by Antisell ' as follows : 

Standing at the foot of these [Bighorn or Goat] mountains and looking backward over the trail traveled, the character 
of the country becomes apparent; it is an immense extended plain as far as the eye can reach (about 60 miles), 
sloping slightly to the southwest, and equally level north for 35 miles, the horizontality only disturbed by the isolated 
hills or ranges described, whose general direction is N. 60® W. The soil of the plain is uniform — a feldspathic or 

I Spurr. J. E., Descriptive geolof^ of Nevada south of the fortieth parallel and adjacent portions of California: Bull. U. S. Geol. Survey No. 208, 
19(». 

> Mo<}ee, W J, Sheetflood erosion: Bull. Geol. Soc. America, vol. 8, 1897, p. 89. 

* Antisell, Thomas. Repts. Expl. and Surveys for Railroad from MissisBippi River to the Pacific Ocean, vol. 7, pt. 2, 1856, p. 133. 



GEOGRAPHY. 15 

granitic sand, with occasional drifts of fine quartz sand, easily impressed by the hoof of the beast, and brilliant, so aa 
to pain the eye in midday by the reflection of the sunlight. 

There are between 50 and 60 mountain ridges in the district, so that the area covered 
by the average ridge is about 100 square miles. Fully half of them, however, occupy an area 
averaging less than 50 square miles to a ridge. Their ratio of width to length is 1 : 4.5. Like 
the ranges to the northeast, the ridges show a general parallelism in trend, which is in a north- 
westerly direction. They are much more scattered and more highly dissected than the ranges 
in the Mountain district and have aptly been called ''buried mountains.^' 

GEOGRAPHY OF THE VOLCANIC FIELD. 

TOPOQB.APHY. 

In the more distant views of the volcanic field one is impressed only by the large cones 
and their complete isolation from one another. From all points of view San Francisco 
Moimtain stands out with great distinctness, rising with graceful outline to a height of 12,700 
feet above the sea, or over 5,000 feet above the surroimding country. Kendrick Peak, 
11 miles to the northwest, is second in size and has an elevation of 10,500 feet above sea level, 
or 3,500 feet above its base. Bill Williams Mountain and Sitgreaves Peak, in the west'em 
part of the region, rise not more than 9,500 feet above the sea, or 2,500 feet above the adjacent 
country. O'Leary Peak and Elden Mountaiil, situated closer to San Francisco Mountain on 
the east and southeast, have elevations of about 9,000 feet and rise 2,000 feet above their 
bases. Mormon Mountain occupies a somewhat isolated position 28 miles south of San Fran- 
cisco Moimtain. It is the smallest of the lai^e vblcanoes, for its elevation is 8,600 feet above 
sea level, or 1,500 feet above the surroimding region. Small volcanic cones are scattered 
about with great irregularity; at some localities they are grouped closely together, at others 
they are entirely asbent. To a person riding among them they appear to be rather insignificant 
features of the landscape as compared with the large cones, for few of them attain heights of 
more than 700 feet. It is only when they are viewed from the commanding summits of the 
higher peaks that they are seen to produce considerable detail in the topography. Although 
the surface slopes throughout most of the field are gentle, they bring about notable changes 
in elevation over long stretches. This may be best appreciated from the general geologic 
map (PL III, p. 20) which has a contour interval of 1,000 feet. 

The surface of the country south of the railroad, from the vicinity of Flagstaff to the 
western boundary of the field, is fairly level and stands at an elevation of 6,500 to 7,000 feet. 
The field is abruptly terminated on the west and south by the precipitous Aubrey CliflFs, which 
gradually increase in height from 500 feet near Williams to more than 1,000 feet at Oak Creek. 
Bill Williams Mountain is the only large volcano in this part of the region and small cones 
are less numerous than in any other locality except the extreme eastern part. Several canyons 
have been cut back from the Aubrey Cliffs into the plateau. Those of Sycamore and Oak 
creeks on the south are the largest. They are 1,000 and 1,500 feet deep, respectively, and about 
8 miles long. 

The greater part of the region north of the railroad and west of San Francisco Mountain 
has much the same general elevation as that to the south, although it is somewhat higher in 
the vicinity of the large cones. Northwest of a line joining Bill WilUams Mountain and Red 
Mountain, at the head of Hull Wash, the surface slopes gently down to 6,000 feet at the head 
cahyons of Cataract Creek. Kendrick and Sitgreaves peaks, as well as many small cones, 
are situated in this part of the field. They introduce into the landscape a noticeable diversity 
of relief, which is reflected in a broader and more subdued form in the many lava flows. 

The country immediately north of San Francisco Mountain and as far west as Kendrick 
Peak has an elevation of about 8,000 feet. The general surface does not maintain this eleva- 
tion, however, but slopes down gradually to the north at an angle of IJ®, until at the edge 
of the mesa overlooking Hull Wash, it has an altitude of 6,700 feet. A number of basalt 
cones are situated in the neighborhood of the large volcanoes, but farther north they are 
absent and the surface has only the minor relief produced by lava flows. 



16 SAK FRANCISCAN VOLCANIC FIELD, ARIZONA. 

East of San Francisco Mountain is a thickly clustered group of small cones known as 
the Black Hills, and in the northeastern part of the field is another group, perhaps equal in 
number but more widely scattered. In general appearance, however, the country is much 
the same as elsewhere in the field and may be described as consisting of a gently rolling surface 
whose continuity is broken by numerous conical and dome-shaped hills, few of which exceed 
1,000 feet in height, and here and there by a cone of large size. (See PL I, A.) The entire 
eastern part of the field differs considerably in elevation from that to the west. Although 
having an altitude of 7,000 feet in the vicinity of San Francisco Mountain, it drops off with 
marked uniformity and at Little Colorado River, distant 20 miles, it stands at 4,500 feet. 

The southeastern part of the area described in this report, in contrast to the rest of the field, 
presents an unbroken expanse of sedimentary strata. The surface is wholly lacking in relief, 
though cut by many washes and canyons, of which Canyon Diablo is the lai^est. It slopes 
gently northeastward f{pm an elevation of 7,000 feet on the east side of Black Mesa to 5,000 
feet at Little Colorado Kiver. 

DRAIN AGE. 
GENERAL CHARACTERISTICS. 

The San Franciscan volcanic field may be divided into three large drainage areas, but 
all the surface water eventually finds its way into Colorado River. The eastern half of the area 
is drained by the Little Colorado, and the northwestern one-eighth drains into Cataract Creek, 
both of which streams join the Colorado in the Grand Canyon. The southwestern three-eighths 
drains into Verde River and, through the Oila, finally into the Colorado at Yuma, 60 miles 
from its mouth. 

The drainage system of the region is very simple and consists of about 12 principal water 
courses or washes radiating from the higher parts of the field. Within the lava field the courses 
of these washes are not everywhere well marked and have frequently been modified by suc- 
cessive lava flows, but in the sedimentary rocks they are well defined locally as shallow valleys, 
more generally as precipitous walled canyons. 

The common characteristic of all these washes and canyons is their dryness. After a 
he^avy shower or cloudburst the wash draining the area on which the rain has fallen carries 
water for a short time and then resumes its accustomed dryness. Oak Creek is the only per- 
ennial stream within the borders of the lava field. It is fed by several large springs that break 
out at the intersection of two fault planes at the head of the canyon 500 feet below the surface 
of the surrounding country. 

The absence of perennial streams is a serious obstacle to travel and makes it necessary 
to depend for water on springs, lakes, and water pockets, or ''tanks,'' as they are called in the 
Southwest. Springs furnish the most reliable supply and the larger ones may be considered 
permanent, as they have not failed since the region was opened, over 30 years ago. The smaller 
springs, as well as the lakes and tanks, are transient; their existence depends on a very rainy 
season or series of seasons. 

Little is known concerning the underground drainage of the region, although the great 
extent of the limestone formations and the presence at many points of "bottomless" pits and 
fissures indicate that it may be of some importance. Gilbert ^ has described such a fissure 
at the McMillan ranch, near Canyon Diablo, in the Little Colorado Valley. This fissure con- 
tains running water at a depth of 100 feet and is supposed to be due to tension in the brittle 
limestone, as there are several small faults of 10 to 50 feet throw in the region. A similar 
fissure may be seen on the Tuba road, some 12 miles west of the Little Colorado. Several 
pits due to the solution of the limestone by percolating surface waters were observed, such 
as the Bottomless Pit (PI. II, 5), on the road to the cave dwellings in Walnut Canyon. This 
pit is situated in a small sUt-filled valley and has been but recently opened. The underground 
drainage is not yet well established, for the amount of water brought to it in the spring is some- 

1 Gilbert, O. K., A rock flasare: Sctonoe, vol. 2, 1805, p. 117. 




A. BLACK BILL PARK, EAST OF SAN FRANCISCO AND ELDEN MOUNTAINS. 
Showing parklilte character of the country in the pine lone at 7,000 feet and scatteretf cones. The do< 
in center is OLeary Peak. Sunset Peak is next on the right. 




Looking north fro 



OAK CREEK CANYON. 

jsence of a fault, which has controiie 
) in the elevations of the opposite wal 




A. COCONINO MONOCLINE AT COCONINO POINT, LITTLE COLORADO VALLEY. 

Looking north. 




n. BOTTOMLESS PIT, SOUTH OF ELDEN MOUNTAIN. 
Showing development of undergcound drainage. Photograph by A, E. Hackett. Flagstaff, Aci, 



GEOGRAPHY. 



17 



times too great to be carried oflf and a temporary lake is then formed, such as commonly existed 
before the pit was developed. A similar depression is reported to exist at the south end of the 
Bellemont Prairie, and the deep, steep-sided lake sites of the Black Mesa, such as Stonemans 
Lake, have presumably resulted from the solution of the underiying limestone. 

SPECIAL CHARACTERISTICS. 

Lava flows have naturally caused numerous minor changes in the drainage system of 
the region. The damming of watercourses by flows has given rise to many small lakes. Some 
of these still persist; others have been drained by the cutting down of the obstruction that 
formed them, although not before they had been more or less filled with sediment. The grass- 
covered glades, which are picturesque features of 
the landscape throughout the pine forest, generally 
indicate the location of former lake sites. Many 
of the open spaces, especially the larger ones, are 
not, however, of this nature. Two typical exam- 
ples of drainage modification by lava flows may be 
seen on Little Colorado River at Black Falls and 
Grand Falls. 

The Ijittle Colorado for several miles above 
and below Black Falls occupies a shallow valley 
into which a small isolated lava stream was poured, 
covering an area of about 3 square miles and hav- 
ing a thickness of possibly 25 feet or more. This 
flow ponded back the river for a distance of . 2 
miles, forming a shallow lake, and made it seek a 
new course, nearly in line with its former channel, 
partly across and partly along the side of the lava 
flow. Since the obstruction occurred the river has 
cut a channel in the lava 100 feet wide and 10 feet 
deep for 200 feet above the downstream end of the 
flow. At the head of this channel the river cas- 
cades over the bare lava for a short distance with 
a drop of less than 10 feet, but above the so-called 
falls it flows on the surface of the lava at practically 
the same, grade it had before the damming took 
place. As may be seen, the changes at this locality 
have been very slight. 

The phenomenon at Grand Falls is on a much 
larger scale and possesses greater interest. (See 
fig. 3.) Here the river formerly flowed in a perpen- 
dicular-walled canyon, 125 feet deep, cut in lime- 
stone. A lava stream, following the course of a 
side wash, reached the river, completely filled the canyon, and spread over the country east of 
the river for a quarter of a mile. The lava also ran down the canyon for 1^ miles, filling it 
from side to side in a wedge-shaped mass. It must also have filled the canyon in the same man- 
ner above the point of inflow, though this part is not now exposed. 

A narrow lake was formed on the upstream side of the lava flow, in which sediments were 
deposited and all traces of the former canyon liidden. Occasionally, however, when the river 
is in flood and strongly scouring, the west edge of the canj^on is exposed for short distances. 
This lake must have been very transient, for the cutting down of its outlet in the soft red shale 
and sandstone that overlie the limestone could not have taken more than a comparatively few 
vears. The river established a new channel aroimd the east end of the lava flow and fell into 

75008'— No. 76—13 2 





- ----'■.-■ 



Basalt 



AUuviiixn 



M 



Mbencopie 
formation 



Uppcorpanof 
Auoreyg;roup 



Figure 3.— Sketch map of Grand Falls, Little Colorado River. 



18 SAN FRANCISCAN VOLCANIC FIELD, AKIZONA. 

the old canyon at a right angle to its former course and just below the mam pomt of obstruction. 
At first the fall was but a few feet, the distance from the rim of the canyon to the surface of the 
lava filling. But as the lava was eroded the height increased, until now, the lava at this point 
having been entirely removed, the fall is equal to the original depth of the canyon — 125 feet. 
The width of the falls is about 400 feet and appears to have been constant throughout the 
process of development. 

The river has made some headway in removing the thin-bedded limestone strata that form 
the upper part of the canyon wall, and it cascades over their eroded edges for a quarter of a mile 
at a grade of 2°. It then falls abruptly over the edge of a thick bed of limestone and after 
flowing on the surface of the underlying stratum for 50 feet again drops vertically to the bottom 
of the canyon. The lowest bed of limestone has not retreated more than 20 feet from the posi- 
tion it originally occupied in the canyon wall. Below the falls about one-half of the wedge- 
shaped .mass of lava remains along the west side of the canyon. 

The exposures at Grand Falls are so complete that the amount of material eroded can be 
very closely calculated. On the other hand, the flow of the river and the* rate at which it erodes 
are wholly unknown. It is not possible, therefore, to determine the age of the falls, however 
interesting the problem. It is sufficient to say that they are, in geologic terms, distinctly 
youthful. 

CLIMATE. 

The climate of the San Franciscan volcanic field is typically that of an arid to semiarid 
region predominantly under solar control, as the cyclonic element does not constitute more 
than one-fifth of the whole. 

The highest temperatures are experienced in the Little Colorado Valley, where the elevations 
are lowest. The mean annual temperature there is about 56°, the average absolute maximum 
is 100-105°, and the minimum is zero. In the central and western parts of the field, at eleva- 
tions of about 7,000 feet, the mean annual temperature is approximately 46°, the average 
maximum is 92°, and the minimum is —13°. Throughout the region insolation is very great 
and produces a wide range in temperature from day to night. At Flagstaff (elevation 6,900 
feet) this range averages 30° for the year, with an average maximum of 36° in Jime and a 
minimum of 25° in February and March. In the Little Colorado Valley, owing to the decreased 
cloud cover and the absence of vegetation, the daily range is somewhat greater. 

There are two wet seasons — one in summer, the other in winter — which are separated 
by much drier periods. The summer season is characterized by local showers due to conveo- 
tional atmospheric currents. In the higher parts of the region many of these storms are severe 
and a few reach the proportions of cloudbursts. In the Little Colorado Valley, however, the 
temperature is so high that the lighter showers are evaporated before they reach the ground. 
Thunderstorms are especially frequent in summer, but have been reported in every month of the 
year. The winter is marked by rainstorms and in the higher parts of the field by snowstorms — 
some due to convectional atmospheric currents, some to cyclonic disturbances. 

The smallest rainfall in the region — ^less than 5 inches a year — occurs in the Little Colorado 
Valley. At WilUams, in the southwestern part of the field, the rainfall amounts to 15 inches, 
and at Flagstaff it is 22 inches. The rainfall at Flagstaff is greater than at any other station in 
Arizona and is influenced, no doubt, by the proximity of San Francisco Mountain. The average 
snowfall at Flagstaff is 85 inches a year and is distributed through all the months except June 
to September. About 70 per cent of the total falls in January, February, and March. The 
largest snowfall is, of course, on San Francisco Moimtain, and not uncommonly a small snow 
field persists through the summer in the ravine in which SnowsUde Spring is located, immediately 
south of the core ridge. The chief characteristic of the rainfall is its extreme variability. The 
difference in the mean annual rainfall at Flagstaff is over 100 per cent; in the Little Colorado 
Valley it may amount to 1,000 per cent — that is to say, in some years practically no rain falls in 
that area. The differences in the monthly precipitation from year to year are even more striking. 
At Flagstaff the wettest month of the year — February, with an average rainfall of 2.65 inches — 
shows a maximum of 8.36 inches and a minimum of 0.28 inch. 



GEOGRAPHY. 19 

FORESTS. 

The wide range in elevation in the San Franciscan volcanic field and the accompanying 
climatic variations bring about striking changes from place to place in the character of the 
vegetation. Although the field is encircled on all sides by a barren and arid region, much of 
which is in fact a desert, the higher parts of the field are clothed in a beautiful forest of juniper, 
pine, and spruce. The abrupt change from desert to forest, so clearly seen in passing from the 
valley of the Little Colorado westward, delighted the early explorers as it delights the traveler 
of to-day. Beale,^ who traversed the region in 1858, remarked that * *No one could pass through 
this coimtry without being struck by its picturesque and beautiful scenery, its rich soil, and its 
noble forests of timber. The view from our camp of this morning is unsurpassed in the world." 

A study of the plant life of this general region by C. Hart Merriam ' led him to divide it 
into seven distinctly marked zones, ranging from subtropical in the Little Colorado Valley to 
Alpine at the summit of San Francisco Mountain. Of these zones, the one known as the pine 
zone and marked by the presence of the yellow pine (Pinus ponderosa) is the most important 
commercially as well as the most beautiful. These pines, which average over 100 feet in height, 
grow between elevations of 7,000 and 8,200 feet and form a magnificent open forest. Throughout 
its extent there is practically no underbrush; in summer the ground is covered with an abundant 
growth of grass, and the forest gives one the impression of an immense cultivated park. 

1 Beale, E. E., Report on wagon road from Fort Smith, Ark., to the Colorado River: H. Ex. Doc. No. 42. 36th Cong., 1st sess., 1860. 
* Merriam, C. H., Results of a biological survey of the San Francisco Mountain region and the desert of the Little Colorado River, Ariz.: North 
American Fauna No. 3, U. S. Dept. Agr., 1880. 



CHAPTER 11. 

REGIONAI. GEOLOGY. 

TOPICS COVERED. 

This chapter is devoted to the non volcanic geologic features of the region — the sedimentary 
rocks, the structure, and the glaciation and alluviation of San Francisco Mountain. The greater 
part of the chapter is devoted to the description of the sedimentary formations and the dis- 
cussion of their origin and correlation with the corresponding formations of the surrounding 
country. The sedimentary'* record is so entirely unrelated to the volcanic record that it is 
advisable to treat the two separately. The youngest sedimentary rocks, except the Quatemarj^ 
alluvial deposits, are of Triassic age, whereas the first lavas were not erupted until Pliocene 
time. The histor}^ of the region during this hiatus from the Triassic to the PUocene will be 
omitted, as observations in the surrounding country are too incomplete to permit a satisfactory 
outline of it to be given. A general geologic map of the region forms Plate III. 

OUTLINE OF GEOLOGIC HISTORY BASED ON THE SEDIMENTARY 

FORMATIONS. 

The oldest rocks of the region are the pure limestones of the Redwall formation, of Missis- 
sippian (lower Carboniferous) and Pennsylvanian (upper Carboniferous) age. The red sand- 
stone of the Supai (** Lower Aubrey ^0 formation, the cross-bedded Coconino (**Upper Aubrey '0 
sandstone, and the cherty Kaibab (^* Upper Aubrey'*) limestone succeed one another in the 
order given and belong to the Pennsylvanian series (upper Carboniferous). These four for- 
mations furnish a record of continuous marine sedimentation in waters often verv shallow and 
at no time ver}'- deep. After the cherty Kaibab limestone was deposited the r^on was raised 
above the sea without appreciable tilting and that formation was subjected to erosion which 
apparently operated only long enough to produce a youthful topography. Upon this surface 
were laid down without apparent discordance in dip the red sandstones and shales of the Moen- 
copie formation, considered Permian, representing flu via tile or shallow- water deposits. Then 
followed the deposition of the Triassic sandstones, shales, and marls, which are separated from 
the Moencopie formation by a slight unconformity due to erosion, without marked discordance 
in dip. The great diversity, both laterally and vertically, in the composition of these beds, 
the presence in them of many petrified trees, and a land fauna at a certain horizon indicate that 
they are continental deposits — that is, they were laid down on a land surface. The Triassic 
rocks furnish the last record of sedimentation in this region, but a study of the siurounding 
country shows that deposition continued much longer and that Jurassic, Cretaceous, and 
possibly Eocene strata once covered the area. These strata, however, have since been entirely 
removed bv erosion. 

SEDIMENTARY ROCKS. 
OCCUIIBENCE AND GENERAL SECTION. 

The sedimentary^ rocks witliin the lava field are exposed at but four localities — at and near 
Garland Prairie south of Maine, a small town on the railroad between Flagstaff and WiUiams, 
and at Marble Hill, Slate Mountain, and Elden Mountain. At these locaUties only Carbonifer- 
ous strata are exposed, and it is therefore necessary to study the younger formations either on 
tlie border of or beyond the lava field. The strata in general dip less than 1°, except where they 
are locally disturbed by monoclinal folding or the intrusion of igneous rock, and over the greater 

20 



REGIONAL GEOLOGY. 



21 



part of the region this dip is northeastward. The thickest continuous sections are exposed on 
the flanks of the laccoliths; elsewhere, on account of the horizontality of the strata, the exposures 
are much thinner, although in the walls of Oak Creek canyon 1,500 feet of beds appear in 
vertical section. The generalized section for the entire region has an approximate thickness of 
2,500 feet. 

Generalized section of sedimentary rocks in San Francisco Mountain region of northern Arizona. 



System and series. 


Group. 


Formation. 


Estimated 
thickness. 


Lithologic character. 


Quaternary. 






Feet. 


Alluvium. 

Moraines which may be of Wisconsin age. 






"Leroux formation." 


400 


LiKht-oolored shales (white, bluish, and pink), 
with some sandstone and calcareous beds. 


Triassic. 


Shinarump. 


"Lithodendron formation." 

"Shinanmip" conglomerate at base. 


3d5 
(3*-50) 


Sandstone, light-colored shales, and "marls. " 
Conglomerate containing much petrified 
wood. 




It 


Moenoopie formation. 


280 


Red to light-brown shales, with some sand- 
stone and calcareous layers. 




• 

a 
S 
'fl 

9 
> 

s. 


AuT)rey. 


Kaibab (" Upper Aubrey") limestone. 


375 


Cherty limestone. 


Carboniferom. 


Coconino (" Upper Aubrey ") sandstone. 


43&-610 


Cross-bedded white or light-yellow sandstone. 


Supai formation ("Lower Aubrey" sand- 
stone and shale). 


670 


Red sandstone and shale. 




1 




Redwall limestone. 


250+ 


Massive gray limestone. 



PALEOZOIC FORMATIONS. 

The Paleozoic era is definitely represented by four conformable formations, all Carbonifer- 
ous. They are, in order of deposition, the Redwall limestone, the Supai formation (*' Lower 
Aubrey"), the cross-bedded white Coconino (**Upper Aubrey") sandstone, and the cherty 
Kaibab ("Upper Aubrey") limestone. The Moencopie formation is tentatively referred to the 
Permian because on lithologic and stratigraphic grounds it appears to be clearly correlated with 
the Permian of Kanab Creek, on the north side of the Grand Canyon in Utah. 

REDWALL LIMESTONE. 

The upper parts of the Redwall limestone, which at the type locality in the walls of the 
Grand Canyon, 60 miles to the north, has a total thickness of about 1,000 feet, are found on the 
east flank of Elden Mountain and at Marble Hill. At both places the limestone has been upturned 
by igneous intrusion and for the entire thickness exposed, about 250 feet at each place, has been 
changed by contact metamorphism to a pure white medium-grained marble. The strata exposed 
at Marble Hill consist throughout of pure limestone. At Elden Mountain, close to the contact 
with the igneous rock at the northwest corner of the eastern sedimentary area, the limestone 
becomes thinner bedded than usual and gives way to red calcareous shale and sandstone. The 
limestone immediately overlying these beds contains many fragments of shale and includes fit 
one point a patch of conglomerate containing angular to subangular pebbles of quartzite from 
one-fourth to one-half inch in diameter. 



22 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

A specimen of marble from Elden Mountain analyzed by F. N. Guild, of the University of 
Arizona, has the following composition : 

Analysis of marble from Elden Mountain. 

SiOj 0.30 

AljO,, FeaO, 62 

MgCOa 3.25 

CaCOg 96.58 

100.75 

The analysis shows that the specimen is a very pure limestone, which may be regarded as 
typical of the formation in this vicinity. 

Poorly preserved fossils are found in the marble, of which the following from the uppermost 
strata at Elden Mountain have been very kindly determined by Prof. Henry S. Williams, of 
Cornell University: 

Spirifer forbesi near increbesceiiB (Hairs species in Iowa report). 
Derbyia keokuk. 
Orthis (small species). 
Corals (species?). 

On lithologic and stratigraphic grounds these beds are without question correlated with the 
Redwall limestone of the Grand Canyon. The age of the upper part of this formation is Penn- 
sylvanian (upper Carboniferous) . The lower part, which is not exposed in the region, is of Mis- 
sissippian Gower Carboniferous) age. 

The purity of the limestone and the character of the fossils indicate that the formation was 
deposited in oceanic waters, presumably of greater depth than 100 fathoms, as in general shore 
material is absent. The presence, however, of local bands of shale grading into limestone and 
of conglomerate containing angular to subangular pebbles shows that conditions of sedimenta^ 
tion were not uniform. These changes, according to prevailing ideas, would be interpreted as 
showing fluctuations in the elevation of the ocean bottom which at one period brought it within 
the littoral zone and permitted the deposition of the shale and conglomerate. Recent studies * 
have shown, however, that certain conditions of erosion on the land would allow the material 
that formed limestone to be deposited in waters of less than oceanic depth and that changes in 
the character of the sediments may be due to climatic variations. 

AUBREY GROUP. 
BUBDIVISIGHS AlTD HGlEZirOUlTlTBZ. 

New geographic names have recently been introduced by N. H. Darton^ for the subdivisions 
of the Aubrey group, which is typically developed in northern Arizona. The use of the term 
Aubrey to cover the group and also to designate its several subdivisions is contrary to Survey 
rules of nomenclature, hence the necessity for the new names introduced by Darton, which will 
be used in this report, as follows: 

Kaibab limestone (replaces ''Upper Aubrey" limestone). 
Coconino sandstone (replaces "Upper Aubrey" sandstone). 
Supai formation (replaces "Lower Aubrey" sandstone and shale). 

The type localities of these formations are in northern Arizona, near the region covered by 
this report. 

SUPAI FOBKATIOH ("LOWZB AUB&ET" RED SANDSTONE AND SHALE). 

The Supai formation outcrops in the lower walls of Oak Creek canyon and is the surface 
rock in that part of the Verde Valley between Oak and Sycamore creeks known as *'the Red 
Rock country.'' It is also found on the eastern flank of Eklen Mountain and at Marble Hill. 

1 Barrel!, Joeeph, Relations lietween climate and terrestrial deposits: Jour. Geology, vol. 16, 1908, pp. 159-190, 22&-295, 36a-8S4. 

* Darton, N. II., A reconnaissance of parts of nOTthwestwn New Mexico and northern Arizona: Bull. U. 6. Geol. Survey No. 435, 1910. 



J. S. GEOLOGICAL SUHvev 



PROFESSIONAL PAPER 76 PLATE l\ 





BEGIONAL GEOLOGY. 23 

At these localities it is throughout a rather poorly cemented, uniformly fine-grained sand- 
stone, generally light red in color but locally white, and not unconmionly having a cross-bedded 
structure on a small scale. In Oak Creek canyon the upper portion of the formation has much 
the same character as at the points noted above and consists of many thin beds which tend to 
form graded slopes. In the bottom of the canyon, however, the strata are much thicker and 
more resistant, and according to J. W. Fewkes ^ the same conditions exist in the Red Rock 
country. 

The difference in resistance to erosion shown by different parts of the formation depends 
closely on the nature of the cementing material. In the softer beds this material is predomi- 
nantly calcareous; in the harder beds it is siliceous. 

The thickness of the formation, as measured at Marble Hill, is 670 feet, although minor 
faulting due to laccolithic intrusion makes the measurement somewhat uncertain. It agrees 
very closely, however, with that observed by Gilbert ' (600 feet) in the Aubrey Cliffs at a point 
15 miles southeast of Bill Williams Mountain, where the strata are undisturbed. 

The formation has been referred to the Supai (*' Lower Aubrey") purely on lithologic and 
stratigraphic grounds, as no fossils were found in it. Its age is Pennsylvanian (upper Carbon- 
iferous), as determined by fossils occurring in intercalated beds of limestone at several localities 
in the Plateau country. 

Uniform conditions of sedimentation in shallow water in this general region are shown by 
the homogeneous character of the formation. That the beds were laid down in the open sea 
appears to be proved by the limestone strata containing marine fossils at neighboring localities. 

cocoimro ("uppsb aubket* ) sahdstone. 

Extensive outcrops of the Coconino sandstone occur in the Aubrey Cliffs along the southern 
edge of the plateau, in the walls of Sycamore, Oak, and Walnut canyons, and at Marble Hill, 
Slate Hill, and Elden Mountain. The formation throughout is massive and made up of many 
beds of uniform whitish to light-yellowish sand cemented by silica. Its striking feature is cross- 
bedding, which extends through its entire thickness. This feature is illustrated in Plate IV, B^ 
showing the upper 100 feet of the formation as exposed in the walk of Walnut Canyon. The 
dip of the cross-bedding planes ranges from zero to a maximum of 2A? and is generally not 
tangent to the underlying surface of deposition. Its direction, as determined by a number of 
observations, is southward. 

The thickness of the formation was measured at Marble Hill, where it is 435 feet, and at 
Oak Creek canyon, where it is 610 feet. The formation was measured by Gilbert • in the Aubrey 
Cliffs, at a point 15 miles southeast of BiU Williams Mountain, and was found to be 700 feet. 

The formation is unfossiUferous, but is evidently of Pennsylvanian age, as it occurs between 
the Supai formation and the cherty Kaibab Umestone, both of which, on the evidence of fossils, 
have been assigned to the Pennsylvanian. 

The physical conditions under which these strata were laid down are believed to have 
been similar to those existing during the deposition of the Supai formation. The somewhat 
coarser texture and more strongly marked cross bedding indicate more powerful currents 
and possibly shallower water. It might be supposed that the formation was of eolian origin 
on account of the character of the stratification, but the observations made do not appear 
to support this idea. The plunging layers do not generally exhibit the tangency to the underlying 
surface commonly observed in eolian deposits; the maximum angle of dip — 24° — ^is much 
less than the natural angle of slope for dry sand — about 33® — and suggests the presence of 
water. In the absence of conclusive evidence of its eoMan origin, it seems reasonable to suppose 
that the formation is marine, as it hes between two formations of known marine origin. The 
correctness of this opinion is rendered more probable by the fact that the formation in the 



1 Archeological expedition to Arizona in 1895: Seventeenth Ann. Rept. Bur. Am. Ethnology, 1898, pt. 2, pp. 550-509. 
« Gflbert, 0. K., U. S. Geog. Surveys W. 100th Mer., vol. 3, 1875, p. 163. 
I Op. cit., p. 163. 



24 SAN FBANCISCAN VOLCANIC FIELD, ABIZONA. 

Zuni Mountains of New Mexico, where it has the same characteristics as in the San Francisco 
Plateau, contains several intercalated beds of limestone carrying typical marine fossils.^ 

iTAniATi ("ITPPEB AUBREY ") LIMESTONE. 

The Kaibab limestone is extensively exposed on the north, northwest, and southeast sides 
of the volcanic field, where it forms the surface rock of the San Francisco Plateau. It is also 
exposed as the uppermost formation in the face of the Aubrey Cliffs from a point west of Wil- 
liams as far east as Oak Creek. Where it occurs within the lava field it forms the surface upon 
which the lavas rest. It thus seems probable that it underlies the larger part of the volcanic 
area. 

The formation is extremely variable in composition, although, broadly speaking, it appears 
to show a transition from rather impure limestones at the base to much purer beds in the middle 
and again to increasingly impure beds toward the top. It was most probably deposited in very 
shallow waters. The chief adulterant of the limestone is silica. In the vicinity of Flagstaff 
the silica occurs in the form of chert nodules, from 1 to 3 inches in diameter, containing fossil 
sponges, and these nodules, as well as quartz geodes, are abundant in the upper part of the 
formation. On the east side of Anderson Mesa the rock is an arenaceous limestone; at Grand 
Falls, on the Little Colorado, it is even more impure and should probably be classed as a cal- 
careous sandstone. The rock also contains, in addition to the silica, a highly variable and 
sometimes large amount of dolomite. These features are shown by the following analyses, 
by F. N. Guild, of specimens from the upper part of the formation in the vicinity of Flagstaff; 

Analyses of the Kaibab limestone. 



8iO|... 
AltOi.. 
FeiOi.. 
MgCOi 
CaCOi. 




100.02 



1. Canyon southeast of Flagstaff , near mouth of sewer. 

2. Vicinity of Elden Mountain. 

The formation as a whole is white to grayish in color and rests directly on the cross-bedded 
sandstone below. In its upper and lower parts it is rather thin bedded, but near the middle 
it includes two or more beds that are 10 to 25 feet thick, and this sequence appears to be fairly 
constant in the southeastern part of the region (PI. IV, A), 

At no point was the entire thickness of the formation seen. The upper surface is generally 
one of erosion, and at localities where the contact with the overlying formation is found that 
with the underlying formation is hidden. The greatest thickness of the limestone is exposed 
on the east and west sides of Anderson Mesa. On the west side, at a point 2 miles south of the 
Ice Caves, the measured thickness is 340 feet. The full tliickness, estimated from the restored 
section, is believed to be not more than 375 feet. Other measurements, made in Oak Creek 
canyon, in the shallow canyon 2 miles south of Hull Mountain, on the north flank of Elden 
Mountain, and at Marble Hill, range from 220 to 320 feet, the average being 260 feet. It 
would appear, therefore, that about one-tliird of the formation has been removed by erosion. 

Fossils occur in all parts of the formation, and the characteristic ones have been de- 
scribed ' as — 

Productus ivesii. 

P. semireticulatua. 

Spirifer lineatus. 

Aviculipecten, species closely allied to A. occidentalia. 

Athyris sublilita. 

Meekella striatocostata. 

Hemipronites (species?). 



' Dutton, C. E., Mount Taylor and the Zunl Plateau: Sixth Ann. Rept. U. S. Geol Survey, 1S85, p. 132. 
• r. S. Geog. Surveys W. 100th Mer., vol. 3, 1875. p. 177. 



BEGIONAL GEOLOGY. 



25 



Freeh * has described the following fossils from WaLiut Canyon, southeast of Flagstaff. 

Productufl ivesii (very common). 
P. aff. scabriculus (rare). 
Spirifer (Martinia) lineata (rare). 

On this evidence the formation has been assigned to the Pennsylvanian (upper Carbonif- 
erous) epoch. 

COMPARISON OF PENNSYLVANIAX FORMATIONS IN SOUTHERN PLATEAU COUNTRY. 

The red sandstone of the Supai formation, the cross-bedded Coconino sandstone, and the 
cherty Kaibab limestone, which comprise most of the Pennsylvanian of the southern Plateau 
country, appear to be sufficiently diflferent from one another to permit a comparison between 
sections measured at several points. Observations are as yet much too scanty and the sections 
far too scattered to permit any exact conclusions^ although they appear to furnish a basis for 
one or two reasonable inferences. 

The sections measured at the following localities will be used: (1) Generalized section, San 
Franciscan volcanic field ; (2) Hance trail, at the Grand Canyon of the Colorado * ; (3) Kanab 
Creek section on north side of Grand Canyon *; (4) mouth of Grand Canyon at Grand Wash •; 
(5) Zuni Mountains in New Mexico.* The distances and directions in which the sections lie 
from the central part of the San Franciscan volcanic field are as follows: Hance trail, 65 miles 
north; Kanab Creek, 110 miles north-northwest; mouth of Grand Canyon, 150 miles west- 
northwest; Zuni Moimtains, 200 miles east. 

Thickness of formations of the Aubrey group. 



Kaibab limestone . . , 
Coconino sandstone. 
Supai sandstone 



1 


2 


3 


4 


375 


aSOO 


800 


a200 


610 


400 


50 


300 


670 


1,000 


1,455 


800 



400 
800 



a Top eroded. 

The cherty Kaibab limestone decreases noticeably in thickness both southward and 
westward from Kanab Creek. It is possible that the thinness of the limestone at the mouth 
of the Grand Canyon is due entirely to erosion, although this does not appear probable in view 
of the fact that not more than half of the formation has been so removed in the San Franciscan 
volcanic field. The cross-bedded Coconino sandstone, on the contrary, increases very markedly 
southward and westward from Kanab Creek, whereas the red sandstone of the Supai formation 
decreases in all directions from the same point. 

In the Zuni Mountains the red sandstone of the Supai formation preserves its individual 
character, but the cross-bedded Coconino sandstone and cherty Kaibab limestone are no longer 
distinct from each other and are included by Dutton* in one formation, which he describes 
as follows: 

The upper Aubrey is composed laigely of sandstones. In color they are yellowish brown and the cement, 
instead of being calcareous, is siliceous, in fact, a regular chert. These sandstones are often conspicuously cross- 
bedded. * * * Intercalated with them are three or four thick beds of pure limestone containing an abundance of 
fossils of many and characteristic species. 

The formation is evidently very similar to the Coconino C' Upper Aubrey") sandstone as 
it occurs in the western part of the Plateau country, except for the limstone strata. The absence 
of the cherty Kaibab limestone may be explained by erosion, by the thinning out of the forma^ 
tion, or by loss of identity through a change in the nature ot the material. It is probable 

> Freeh, F., Compt. rend. 5th sess.. Cong. rA)1. internat., 1«91. p. 478. 
' Waicott, C. D., Am. Jour. Sci., 3d ser., vol. ao, 1880, pp. 221-225. 

- Gilbert, O. K., op. dt., p. 162. 

- Dutton, C. E., op. clt., PI, XVI, opposite p. 136. 

- Op. cit., p. ia3. 



26 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

that the explanation lies in the last two causes. It was noted in the preceding description of 
the formation that the strata at Grand Falls, on Little Colorado River, are arenaceous and 
should be classed as calcareous sandstones. Likewise at Coon Mountain, a point farther east 
than Grand Falls, the formation, according to Barringer,^ consists of '^ 200 to 350 feet of yellowish- 
gray calcareous sandstone, which when eroded and weathered has the appearance of a limestone. '' 
It is evident, therefore, that the formation becomes more arenaceous in an easterly direction. 
Marvine ' observed the upper members ol the formation in the vicinity of Sunset Pass, 25 miles 
southeast of Canyon Diablo, but states that on Silver Creek, south of Holbrook, a point still 
farther east, '*the overlying cherty limestone was not observed." It apparently loses its iden- 
tity, then, as a limestone formation in the region between Clear and Silver creeks. 

The cherty limestone is about two-thirds as thick at Coon Moimtain as it is farther west. 
But a well record at Winona,' a station on the railroad west of Canyon Diablo, shows that the 
cross-bedded sandstone has decreased in thickness to 456 feet, compared with 610 feet in the 
San Franciscan volcanic field. Thus it would seem as if both the crosa-bedded sandstone and 
cherty hmestone become thinner to the east and coalesce, constituting one formation, 400 feet 
thick, in the Zuni Moimtains. 

The facts above stated possibly indicate the deposition of the two formations on a continental 
shelf whose shore line was situated east of the Zuni Mountains. It is at present difficult to 
interpret the alternate thinning and thickening of the three Pennsylvanian formations from 
the vicinity of Kanab Creek southward. The first inference would be that this locality was 
offshore and that the sea became shoaler toward the south. This view seems to be opposed by 
the fact that the Pennsylvanian rocks in southern Arizona are predominantly represented by 
a limestone formation concerning which Ransome^ says: 

[It] was deposited in moderately deep water at some distance from the shore. * * * During certain stages of 
the accumulation of the limestones offshore currents carried some of the finest of the land waste into this area of tranquil 
deposition and left records of these occasion il incursions in the form of pink shales. 

TBIASSIC AND PERMIAN (?) BOCKS. 

The Shinarump group, wliich in this area is for the present divided into the Moencopie for- 
mation, the ''Lithodendron formation/^ and the '^Leroux formation/* is mapped as ''Triassic 
and Permian(?).'' The Moencopie formation is believed to be Permian; the others are Triassic. 

SHINARUMP GROUP. 

The Shinarump group is exposed at several localities near the edge of the lava field, as in 
the vicinity of Flagstaff, on the north and east sides of Anderson Mesa, in the walls of Sycamore 
Creek canyon, in the face of tbe mesa about Cedar Ranch, and east of the lava field, where it 
in part forms the surface rock of the Little Colorado Valley. 

The most complete section observed (given below) is in the outliers and face of the mesa, 
mostly about half a mile south-southeast of Cedar Ranch, the locality containing the outcrop 
of petrified wood. Divisions 1 and 2 occur in the low liills at the northeast corner of the mesa; 
the others are in the face of the mesa a short distance south. The section is broken between 
divisions 2 and 3, but has been adjusted so as to be continuous. The base of the section is the 
cherty Kaibab limestone, upon which the shales rest in apparent conformity. 

Section near Cedar Ranch. 

Feet. 

10. Basalt. Rests on eroded surface 50 

9. Alternating pale-red. pale-lavender, or gray shales and sandstones in strata from 10 to 20 feet 

thick. All weather easily. Upper part not clearly exposed 135 

8. Whitish to pale-reddish firm sandstone 25 

7. Whitish to pale-redd Lih, rather soft arenaceous shale, in places conglomeratic and containing 

a small amount of petrified wood 85 

I Barringer, M., Coon Mountain and ita crater: Proc. Acad. Nat. Sci. Philadelphia, vol. 57, pt. 3, 1905, pp. 864-^<65. 

« Marvine, A. R., U. S. Geog. Surveys W. 100th Meridian, vol. 3, 1875, p. 213. 

 Barringer, M., op. cit., p. 865. 

4 Ransome, F. L., The geology and ore deposits of the Bisbee quadrungle, Arizona: Prof. Paper U. 8. Oeol. Survey, No. 21, 1904, p. 46. 



BEGIONAL GEOLOGY. 27 

Feet. 

6. Light-gray slightly arenaceous marl 85 

5. Yellowish to white medium to coarse grained sandstone containing rounded pebbles up to 
3 inches in diameter, many angular fragments of petrified wood, and in places an entire 
section of a tree 35 



365 



4. Red shales with a mottled red and gray calcareous clay layer 2 feet thick and a gray calcareous 
sandstone 6 inches thick at the base. The clay contains 68 per cent and the sandstone 75 
per cent of material insoluble in hydrochloric acid 70 

3. Red shales; at base several layers of gray calcareous shale from half an inch to 8 inches in 

thickness, of which 64 per cent is insoluble in acid 25 

2. Light-brown to dull-red soft shales, with a few harder beds. All thin and weather easily. 
At base is a layer of gray calcareous sandstone 1 foot thick, of which 60 per cent is insol- 
uble 85 

1. Bright-red shales, strongly ripple marked ; in upper part is a thin stratum of white fine-grained 

calcareous sandstone, somewhat cross-bedded 100 



280 



Total thickness of section 695 

The strata throughout are horizontal and no unconformities due to erosion were observed. 
The small area of the exposures would make the observation of such unconformities unlikely in 
any locality. The section is rather clearly divisible into two parts. The lower 280 feet consists 
of red and brown shales, in places arenaceous, containing a number of thin intercalated cal- 
careous beds; the upper 365 feet is made up of much lighter colored marls, arenaceous shales, 
and sandstones, at the base of which is a coarse sandstone containing many pebbles and much 
petrified wood. The significance of these features will be discussed later. 

At Flagstaff not more than 25 feet of red shale is present between the capping basalt and 
the Kaibab limestone, but on the south slope of the mesa, 2 miles northeast of town, the thick- 
ness increases abruptly to 150 feet. Red shales form the lower part of the section at this point 
and red sandstones the upper part. The difference in the thickness of the beds in this vicinity 
is due to irregularities in the surface of the underlying cherty limestone and represents an 
unconformity by erosion. 

The composition of the rock from the quarry northeast of Flagstaff, which shows it to be a 
slightly calcareous sandstone, is as follows: ^ 

Analysis of rock from quarry northeast of Flagstaff. 



SiOj 79.19 

AljO 

FeAf ^'^^ 

FeO 



MgO 3.20 

CaO • 7.76 

H20(orloe8) 3.26 



97.16 

On the east side of the nortli end of Anderson Mesa a thickness of about 400 feet of red 
shales and sandstones is exposed, and at the base is 5 feet of fine-grained red conglomerate. 
The upper part of the section is weathered to a fairly well-graded slope, but from the large 
quantity of pebbles near the top it is evident that a conglomerate corresponding to division 5 
of the Cedar Ranch section is present, although petrified wood does not occur at this point. A 
quarter of a mile farther south the thickness of the section has increased to 550 feet, which 
indicates an unconformity by erosion without discordance of dip. 

In the upper walls of Sycamore Canyon from 300 to 400 feet of the red beds are exposed. 
At the top of the section, on the east side of the canyon, is a conglomerate which from its position 
must be the same stratum that is seen at the other localities. The relation of these red beds to 
the Kaibab C* Upper Aubrey *') limestone was not determined, but they owe their preservation 
partly to being faulted down into the underlying formations. This is the southernmost exposure 
of the Shinarump group thus far recorded in the southwestern part of the Plateau country. 

^ Merrill, G. W, Stones for building and decoration, 1903, p. 420. 



28 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

COMPARISON AND CORRELATION OF TRIASSIO AND PERMIAN (?) FORMATIONS. 

A comparison of the several exposures of the post- Aubrey formations in the San Franciscan 
volcanic field shows that they are all rather similar. In each the lower portion consists of red and 
brown shales, with local sandstones, which at Cedar Ranch contain a number of thin intercalated 
layers of a calcareous nature; the upper portion is made up of light-colored marls and sandstones 
and is separated from the lower portion by a conglomerate locally containing many fragments 
of petrified wood. The upper part is also somewhat gypsiferous, at least to a greater extent 
than the lower. 

This general sequence agrees very closely with that of the sections of corresponding strati- 
graphic position observed by Gilbert,* Walcott,^ and Button ' in the region north of the Grand 
Canyon, and by Ward * in the Little Colorado Valley, and leaves little doubt as to their equiva- 
lence. That portion below the conglomerate containing the petrified wood C'Shinanimp'* 
conglomerate) is therefore correlated with the Permian of Walcott's Kanab section and the 
Moencopie formation of Ward. The upper portion belongs to the ^'Lithodendron formation" 
and the ^'Leroux formation,'' which are of Triassic age. 

On the strength of the fossil evidence discovered by Walcott at Kanab Creek in 1879 the 
strata lying between the cherty Kaibab limestone and the ^^Shinarump" conglomerate, at the 
base of the *'Lithodendron formation," have been tentatively referred to the Permian wherever 
they have been found in the southern Plateau country. It may be noted, also, that this forma- 
tion (Moencopie) is separated from those below and above by erosional unconformities without 
notable discordance of dip, and that the upper unconformity, originally observed by Walcott 
at Kanab Creek, has also been recorded by Button and by Bavis.' More recently Ward,* as 
the result of studies in the Little Colorado Valley, has referred these beds to the Triassic. This 
conclusion is based on a transition, which he observed at several points, between the Moencopie 
and the overlying ''lithodendron formation," and on the fact that although a *' marked uncon- 
formity" exists between the underlying Kaibab limestone and the Moencopie formation, none was 
observed by him between the Moencopie and the *'Lithodendron formation." There appears 
to be sufficient agreement among observers as to the general sequence of the strata above the 
Kaibab limestone, and doubt is expressed only as to the age of the Moencopie formation. The 
divergence of opinion has evidently arisen through the different viewpoints from which the 
problem is regarded — one lithologic, the other paleontologic. Of the two it seems as if the 
latter should be given the greater weight at present. Most of the unconformities in this part 
of the Plateau country are those due to erosion without any notable discordance in dip, and 
they may represent either a great or a small stratigraphic break, the determination of whoso 
extent depends almost entirely on fossil evidence. In the absence of such evidence, there- 
fore, no particular significance can be attached to the presence or apparent absence of such an 
unconformity. 

ORIGIN OF TRIASSIC AND PERMIAN (?) FORMATIONS. 

The Moencopie formation (Permian?), with its red and brown shales and sandstones, here 
and there extensively ripple marked, containing some gypsum and thin lens-shaped beds of a 
calcareous nature, is considered as being a very shallow water deposit in an arid to semiarid 
region. The area of observation, however, was not sufficiently extended to permit a conclu- 
sion as to whether it is fluviatile or estuarine in origin. At Cedar Ranch the intercalated 
calcareous beds become increasingly arenaceous toward the top of the formation, and this is 
interpreted as indicating a progressive shoaling of the waters in which the sediments were 

> Gilbert, G. K., U. S. Geog. Surveys W. 100th Mer., vol. 3, 1875, p. 160. 

s Walcott, C. D., The Permian and other Paleozoic groups of the Kanab Valley, Arizona: Am. Jour. 8ci., 3d ser., vol. 20, 1880, pp. 221-225. 

* Tertiary history of the Grand Canyon district: Mon. U. S. Geol. Survey, vol. 2, 1882, Chapter II. 

< Ward, L. F., Geology of the Little Colorado Valley: Am. Jour. Sci., 4th ser., vol. 12, 1901, pp. 401-413. 

* Davif, W. M., An excursion to the Plateau province of Utah and Arixona: Bull. Mus. Comp. Zool. Harvard Coll., vol. 42, Geol. Ser., vol. 6, 
No. 5, 1904. 

* Op. dt., pp. 400-407. 



BEGIONAL GEOLOGY. 29 

deposited, a view that is supported by the character of the overlying conglomerate. The forma- 
tion thins out southwestward from the Little Colorado Valley, and although this may be due 
to erosion, it is taken rather to point to the location of a shore line, or land area capable of 
supplying waste for the sediments, in southwestern Arizona beyond the present boimdary of 
the Plateau. This view is supported by the change in the character of the intercalated beds 
from limestone at Kanab Creek to calcareous sandstones and other near-shore deposits at Cedar 
Ranch. (See p. 30.) 

The overlying Triassic beds are predominantly, if not entirely, a continental deposit, as was 
suggested by Huntington and Goldthwait * in their description of the Triassic rocks near Toquer- 
ville, Utah. This conclusion is based on the character of the strata and their fossil contents. 
One of the striking features of the rocks is the great diversity of the material composing them, 
both horizontally and vertically. Ward's description' of the so-called ^'Shinarump conglom- 
erate'' Gater changed by him to *'Lithodendron formation"), which applies equally well to the 
overlying *'Leroux formation," illustrates this point. He says: 

Although perhape the most prominent feature of the Shinarump is the so-called congloxneiale, which sometimee 
in truth deserves that name and contains somewhat large but always well-worn pebbles and cobbles derived from the 
underlying formations, still it rarely happens that this aspect of the beds constitutes the major portion of them. In 
the first place, the conglomerate tends to shade of! into coarse gravels and then into true sandstones. They are, more- 
over, always more or less cross-bedded and usually exhibit lines of pebbles running through them in various directions. 
Although the sandstones generally occur lower down, still there is no ui^iformity in this arrangement, and the sand- 
stones are often found in the middle and conglomerates more rarely at the top. But in addition to these the Shinarump 
conglomerate embraces other classes of beds. There is a well-stratified layer of thinnish sandstone shales that is often 
seen inmiediately under the heavy sandstone cap. Some of these shales have a grayish color and are highly argil- 
laceous. These layers tend to thicken even within the formation itself, but especially farther out, and what is more 
significant, they often become transformed into a bluish-white marl. In the Petrified Forest region, where the Shina- 
rump conglomerate attains its maximum thickness of 700 to 800 feet, this tendency on the part of certain beds to become 
transformed into marls is the most marked feature of the formation. The marls here occupy much more than half of 
the beds. They are very varied in color, showing besides the white and blue tints a great variety of darker ones, such 
as pink, purple, and buff. These heavy marl beds are interstratified between conglomerates, coarse gravels, and 
cross-bedded sandstones. * * * It thus becomes necessary to include under one designation all these varying 
beds which often change the one into the other at the same horizon within short distances. * * * In the lower 
Little Colorado Valley there occur numerous somewhat calcareous clay lenses, the lime taking the form of bright white 
stripes, while the clay is usually purple or pink. These are very distinct objects and vary in size from lenses 10 or 
even 20 feet in length to small lenticular blocks or somewhat oval or even spherical clay balls or pellets. 

Vertebrate fossils have been found only in the lower portion of the '^Leroux formation," 
which is about 400 feet thick. They consist of fragmentary remains of a labyrinthodont, two 
species of belodont, a dinosaur, and a cotylosaurian (Placerids hestemus). Concerning the last, 
Lucas ' says: ''Indications are that Placerias was a creature largely if not entirely terrestrial in 
habit.'' 

The same statement may be applied to the dinosaur, referred by Lucas * to Palseoctonus 
Cope. The exact habitat of the labyrinthodont and belodonts may be a matter of doubt, but 
in all probabihty it was either fresh-water streams or swamps. 

Petrified wood occurs throughout the *^Lithodendron formation'' and the *'Leroux forma- 
tion" but is most abundant in the sandstone overlying the variegated marls in which the verte- 
brate fossils are found. In the North Sigillaria forest, discovered by John Muir 9 miles north 
of Adamana, many trees are actually in place, a fact, of course, that bears only one interpreta- 
tion. South of Adamana are three forests separated from one another by intervals of several 
miles, and to the west is a fourth forest. Fossil trees are very abundant in all these other 
forests, but none are in place. It seems not improbable that this local concentration of fallen 

i Huntington, Ellsworth, and Ooldthwait. J. W., The Hurricane fault in the Toquervillo district. Utah: Bull. Mus. Comp. Zool., Harvard 
Coll.. Geol. Ser.. vol. 6, No. 5, 1904. pp. 210-213. 
« Op. cit., p. 400. 

* Lucas, F. A., .\. new batrachian and a new rcptilo from the Trias of Arizona: Proc. U. 8. Nat. Mus., vol. 27, 1904, pp. 193-195. 

* Lucas, F. A., Paleontological notes: Science, vol. 14, 1901, p. 376. 



30 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

trees in closely adjacent areas is to be attributed to log jams in a river course, an idea suggested 
by Veatch's description of the ''Great Raff of Red River in Louisiana.^ 

POST-FENNSYLVANIAN LAND ABEA OF SOTTTHWBSTEBN ABIZONA. 

It will be recalled that the Mississippian and Pennsylvanian formations of the San Fran- 
ciscan volcanic field are supposed to have been deposited in a sea whose depth was becoming 
in a general way progressively less. At the end of Pennsylvanian time the area became a land 
surface, was slightly eroded, and had laid upon it the Moencopie formation (Permian?). The 
thinning out of this formation toward the southwest and certain changes in the character of 
the strata indicate that the waste-supplying land area was situated in southwestern Arizona 
beyond the present boundary of the Plateau. 

The most interesting evidence on this point, however, is furnished by the pebbles in the 
conglomerate at the base of the ''Lithodendron formation." In Sycamore Canyon, at the 
southern edge of the Plateau, many pebbles are distinctly subangular and are composed of 
gneiss, jasper, and other metamorphic rocks as well as basic igneous rocks of granitic texture. 
Fairly rounded cobbles of sandstone and chert up to 8 inches in diameter are less abundant. 
The greater part of them represent rocks of the same character as are exposed to-day in the 
Bradshaw Moimtains. At Cedar Ranch, 40 miles north, there is a notable difference both in 
the shape of the pebbles and in the variety of rocks composing them. They are well rounded 
and have been derived mainly from strata that are now exposed only in the Plateau country. 

Rocks similar in nature to those forming the mountain ranges to-day were thus exposed 
in southwestern Arizona, beyond the boundary of the present plateau, at the beginning of 
Triassic time. Areas of the upper formations of the Aubrey group were present in the same 
region, although the subordinate part played by pebbles from these formations in the con- 
glomerate at Sycamore Canyon would seem to indicate that they did not originally extend 
much farther southwest than the present position of the Verde Valley; or that if they did 
cover a larger area, they were of only slight thickness. The subangularity of many of the 
pebbles in the conglomerate at Sycamore Canyon indicates that the source of material was 
close at hand. 

A portion of southwestern Arizona of unknown extent was, therefore, a land area of 
sufficient elevation to supply large quantities of waste to the lower-lying country on the north 
during Triassic time and in all probability during Permian time. It was very probably a land 
area for a much longer period, as the following quotations indicate: 

Ransome ' describes the conditions existing at this time at Globe, Ariz., as follows: 

The upper Carbonifei^uB limestone is the latest Paleozoic deposit of which the region preserves any record. If 
marine conditions continued into the Permian the deposits of that period must have been wholly removed before 
the strata were broken up and invaded by diabase. Had Permian or later beds been involved in that structural 
revolution some trace of them would probably have been preserved in resulting intricate lithologic mosaic. 

Concerning the region about Clifton Lindgren • says: 

The time interval between the [end of the] Carboniferous and the middle of the Cretaceous is not represented 
by any sediments; there is, on the contrary, evidence of an epoch of erosion, for the Cretaceous rests unconform&bly 
on the lower Carboniferous at Morenci, where the upper Carboniferous is not present. 

Similar conditions at Deer Creek are described by Campbell ^ as follows: 

Resting on the Pennsylvanian limestone at every point at which it is exposed in this field is a series of greenish- 
gray sandstone and shale beds which contain coal. * * * At most points these beds appear to rest conformably 
upon the Pennsylvanian limestone, but in the middle field there is a visible imcomformity between the limestone 
and the overlying sandstone and shale. 

Although the evidence regarding the age of these beds is not entirely conclusive, there seems to be a general 
agreement that they belong to the Cretaceous system and presumably were deposited in the later stages of that period. 

1 Vestch, A. C, Qeology and underground water resources of northern Louisiana and southern Arkansas: Prof. Paper U. S. Oeol. Survey, No. 
46.1906. 

s Ransome, F. L., Geology of the Globe copper district, Arizona: Prof. Paper U. S. Geol. Survey No. 12, 1903, p. 109. 

s Lindgren, Waldemar, The copper deposits of the Clifton-Morencl district, Arixona: Prof. Paper U.S. Geol. Stirvey No. 43, 1905, p. 94. 

« Campbell, M. R., The Deer Creek ooal field: Bull. U. S. Geol. Survey No. 225, 1904, p. 240. 



GLACIATION AND ALLUVIATION. 31 



Of the Bisbee region Ransome ^ says: 



With the cloee of the Pennfiylvanian epoch the long era of Paleozoic sedimentation * « * came to an end. 
Orogenic forces became dominant, and the region of the Bisbee quadrangle was elevated above sea level. 

During Triassic and Jurassic time the mountainous country elevated by the poet-Pennsylvanian deformation 
was subjected to erosion. If any sediments were deposited within* the quadrangle during these periods they were 
removed prior to the opening of the Cretaceous and have left no record of their former presence. 

It is evident that the Pinal schist contributed detritus to the basal beds [of the Cretaceous], but as it is probable 
that all the schist within the limits of the quadrangle was covered by the Glance conglomerate before any considerable 
part of the Morita beds was laid down, the land mass that furnished the sands and muds must have been outside the 
area under investigation. The main shore line probably lay to the west of the Mule Mountains. 

The close similarity in the conditions existing at these localities, together with the evidence 
from the San Franciscan volcanic field, makes it quite certain that a land area existed in south- 
western Arizona from the end of the Carboniferous period to the middle of the Cretaceous 
period. Whether this area has since been covered by the sea is difficult to decide ; it has prob- 
ably not been entirely submerged. 

OLACIATION AND ALIjUVIATION OF SAN FRANCISCO MOUNTAIN. 

The former presence of a glacier in the large interior valley on the northeast slope of San 
Francisco Mountain * is clearly proved by moraines, an outwash plain, and polished rock siuv 
faces. Closely related material also occurs on the north slope of the mountain. No glacial 
deposits were observed on the south, east, and west outer slopes. Instead, extensive and 
heavy deposits of alluvium in coalescing fans are present, as may be seen by reference to 
Plates V (p. 40) and VII, B (p. 42). 

The character and distribution of the glacial deposits in the interior valley have been 
described Ify Atwood.' In brief these deposits consist of well-developed medial, lateral, and 
terminal moraines, a till sheet, and an outwash plain. The terminal moraine is situated 2 
miles from the head of the valley, at an ele^^ation of 9,200 feet. Above it the valley is occupied 
by many small dome-shaped hills with intervening swampy areas; below it the surface is 
comparatively smooth and thoroughly drained. At the head of the valley, under San Francisco 
Peak, rock surfaces have been somewhat polished and striated by the ice. On the whole, 
however, ice erosion appears to have been slight, as may be judged by the fact that the sides 
of the valley have the same uniform slopes above and below the terminal moraine. That is 
to say, there are no pronounced cirques at the head of the valley, nor are the walls noticeably 
oversteepened. The glacier at the time of its maximum extension had a length of 1| to 2 
miles, a width at its lower end of half a mile, and a minimum thickness, estimated from the 
height of the terminal and lateral moraines, of 200 feet. 

On the north side of the mountain is a deposit composed of the same material as the 
glacial deposits of the interior valley and the alluvial fans on the outer slopes. It grades 
into the fans along its lower (northern) side. This ridge of material has a length east and west 
across the slope of 1 mile and an average width of three-fourths of a mile. Its greatest thickness 
19 estimated at 300 feet. The upper edge is in contact with the lava-formed slope of the moun- 
tain at a rather uniform elevation of 9,600 feet. The elevation of the lower boundary is variant, 
but gradually decreases from 9,000 feet at the east end to 8,700 feet at the west. It is clear that 
this material was not deposited by a glacier. As the interior valley, with its large catchment 
area, held a glacier only 2 miles long, the less favorably situated northern slope, with almost 
no feeding ground, could not have supported an ice sheet sufficient to bring down the amount 
of material now found at the foot of the mountain. Had glaciers existed on the north side of 
the mountain they would have been confined to the three large ravines and their moraines would 
be appropriately arranged about the mouths of the ravines. No such arrangement of moraines 
exists; instead the belt of material extends without change in form or direction across both 

1 Ransome, F. L., The geology and ore deposits of the Bisbee quadrangle, Arizona: Prof. Paper U. 8. Oeol. Survey No. 21, 1904, pp. 106-107. 

* For a detailed description of San Franclacx) Mountain see pp. 40^; for topographic map see Plate V (p. 40). 

* Atwood, W. W., The glaciation of San Francisco Mountain, Arizona: Jour. Geology, vol. 13, 1905, pp. 276-279. 



32 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

uneroded slope and ravine. The explanation of the origin of this moraine-like ridge is pre- 
sumably to be found in such a process as solifluction/ 

The outer slopes of San Francisco Mountain (PI. V), as well as the other large cones of the 
region, are extensively and heavily mantled with alluvial deposits. The material of these 
alluvial fans is, on the whole, coarse and contains many angular fragments of disintegrated 
but unweathered lava: In general it is indistinguishable from the material composing the 
glacial deposits. The resemblance between the alluvial fans on the outer slopes and the glacial 
outwash plain of the interior valley is most striking. Especially significant is the gradation 
into the alluvial fans of the moraine-like ridge of material on the north slope. There can be 
little if any doubt that the most recent alluvial fans now seen on the outer slopes were formed 
at the same time, and consequently under the same climatic conditions, as the glacial deposits of 
the interior valley. The relation of the deposits 'on the north side of the mountain indicates, 
however, that the greater part of the alluviation of the outer slopes occurred before the glaciation 
of the interior valley. The estimate that 75 per cent of the material eroded from San Fran- 
cisco Mountain is contained in the alluvial deposits on the slopes or about the base of the cone 
shows that there was a period of some length during which climatic conditions were favorable 
to alluviation, whereas it was only during the later part of this period that they were favorable 
to glaciation. In general the streams were not able to remove the waste supplied to them. 

It may be concluded, therefore, that the alluvial fans, in part, were deposited when the 
climate was colder than at present and the precipitation was somewhat greater. The angular 
unweathered lava fragments in the alluvium show that rock disintegration was predominantly 
the result of frost action. The position of the fans, \vith maximum grades of 10°, well up on 
the slopes of the mountain clearly indicates the inability of the existing streams to remove the 
large amoimt of waste supplied to them. In general, the effect of the increased cold was to 
cause a much greater increase in the forces of erosion than in those of transportation. 

It may be noted here that with the return of a warmer climate and possibly smaller rainfall, 
the conditions above described have been reversed. Transportation now overbalances erosion, 
and the alluvial fans are undergoing dissection. At many localities they are cut to their outer 
edges by numerous washes, and the eroded material is being redeposited beyond the original 
limits of the fans on lower and flatter slopes. 

It will be seen that the three kinds of detrital deposits on San Francisco Mountain are closely 
related and that their mode of deposition depended primarily on the position of the locality 
where they occur with respect to the sxm. Thus alluvium was deposited on the outer south, 
east, and west slopes, which were most exposed to the sim. On the less-exposed north slope a 
semiglacial deposit was laid down. In the interior valley, so situated as to receive the least 
direct sunlight, glacial conditions existed and the material was deposited b}' ice* Deposition 
was thus effected by water in the most exposed localities, by water and snow (or ice) m less 
exposed localities, and by ice in the least exposed situations. 

The glacial deposits on San Francisco Mountain, in latitude 35° 20' north, constitute one 
of the southernmost records of ice action within the United States. It will be interesting, 
therefore, to try to form an idea of their age, especially in view of the relation between them 
and the alluvial deposits. The determination of the age of the deposits relative to that of the 
mountain appears simple, but correlation with glacial events as known elsewhere in this country 
must remain in the hypothetical stage because of the isolation of the locality. 

It is most probable that the glacier occupied the interior valley of San Francisco Mountain 
at a time when it had very nearly its present size.^ This is shown by the form of the valley and 
the distribution of the glacial material. If the valley were entirely the result of glacial erosion 
there should be moraines at its eastern extremity. There would also have been glaciers of 
considerable size on the outer slopes, evidences of which should exist in their moraines. That 

^ Andersson, J. G., Solifluction, a component of subaerial denudation: Jour. Geology, vol. 14. 19()6, pp. 91-112. 

« W. W. Atwood, in a personal communication, says: "I do not think that the ice is accountable for very much of the widening or deepening 
of the large interior valley of the mountain." 



GEOLOGIC STBUCTURE. 83 

this would be so is clearly indicated by the glacial conditions now existing on Mount Shasta, 
Mount Hood, and Mount Rainier. No moraine was observed, however, in the interior valley 
below the one situated at an elevation of 9,200 feet, and no true glacial material was seen on 
the outer slopes of the mountain. 

It may be concluded, therefore, as already stated, that the glacier occupied the interior 
valley at a time when it had practically its present size. This shows the recency of the glacier 
and its deposits, as well as the upper (younger) portion of the alluvial fans, both with respect to 
the age of the mountain and actually in geologic time. The freshness of the glacial material, 
even though climatic conditions have been conducive to a minimum of weathering, the undrained 
state of the till sheet and the slight dissection of the deposits all point to the existence of the 
glacier during the Wisconsin epoch, probably the late Wisconsin, of the continental ice sheet. 
A tentative estimate of the postglacial erosion and total erosion of the interior valley gave a 
ratio of 1 : 25. This is considerably greater than the ratio of 1:16 for the erosion since the late 
Wisconsin to that since the Kansan epoch, as suggested by Chamberlin and Salisbury.* If the 
age of San Francisco Mountain is correctly determined as early Pleistocene, this latio of 1:25 
points to the late Wisconsin age of the glacier. 

GEOLOGIC STRUCTURE. 
OENEKAL CHABACTEB. 

The geologic structure of the part of the San Francisco Plateau treated in this report, 
and indeed of the entire region south of the Grand Canyon of the Colorado, is simple and dup- 
licates the features in the district north of the canyon described by Dutton.' The simplicity 
extends only to the nature of the displacements and not to their interpretation, as has been 
shown by the recent work of Davis and of Huntington and Goldthwait. The movements have 
been expressed by both monoclinal and asymmetrical anticlinal folding and by normal fault- 
ing, but not so strongly as in the region to the north. As a rule, the folds have caused greater 
displacements of the strata than the faults, although the faults are more numerous. 

FOLDS. 

Elevations taken on the upper surface of the Kaibab limestone at a number of localities 
indicate that the main structural feature of the region may be called for descriptive conven- 
ience a very flat anticline. The elevations on which the existence of this fold is based are as 
follows : 

Elevations showing anticlinal structure of San Francisco Mountain region. 

Feet. 

Aubrey Cliffs, 11 miles ^'fest of Williams 6, 400 

Cataract Creek, 8 miles northwest of Williams 6, 300 

Garland Prairie 7,100 

Norria Tanks..: 7,100 

Flagstaff 6,900 

Anderson Mesa, north end 6, 800 

Winona, 15 miles east of Flagstaff 6,300 

Hull Wash, 2 miles east of Cedar Ranch 6,300 

Hull Wash, 11 miles west of Little Colorado River. .- 5, 000 

Grand Falls, Little Colorado River 4, 500 

The fold covers the entire field and its axis strikes N. 30° W., passing through Sitgreaves 
Peak. To the north this anticline dies out between the westward-dipping slope of the Coconino 
Plateau and the eastward-dipping slope from the Aubrey Cliffs. These slopes are structural 
in origin and give rise to the broad trough in which flows Cataract Creek. On the south the 
anticline is abruptly terminated by the Aubrey Cliffs. The slopes of the fold are very gentle 

1 Chamberlin, T. C, and Salisbury, R. D., Geology, vol. 3, 1906, p. 414. 

s Dutton, C. E., Tertiary history of the Grand Canyon district: Mon. U. S. Geol. Survey, vol. 2, 1882. 

75008^— No 76—13 3 



84 SAN FKANCISCAN VOLCANIC FIELD, ARIZONA. 

in all directions. The dip of the eastern limb is less than 1^ and is extremely uniform whatever 
the distance over which it is measured, as may be seen from the following table: 

Dip of eastern limb of anticline. 

Minutes. 

Flagstaff to Grand Falls, 32 miles 52 

Cedar Ranch to Little Colorado Valley, 18 miles 48 

Anderson Mesa to Angell, 12 miles 45 

Anderson Mesa to Winona, 7 miles 48 

Anderson Mesa at north end, 3} miles 45 

Anderson Mesa at north end, IJ miles 52 

From the summit of the fold to FlagstaflF, about 18 miles, the slope is 10' E., and from the 
summit westward to the Aubrey CliflFs, 22 miles, it is 20' W. The slope of the crest of the fold 
can not be given, as observations are lacking, but it appears to be nearly horizontal as far north 
as Mount Sitgreaves, from which it pitches gently down to the level of the Cataract Creek trough. 
The slopes given above are the present slopes of the fold. If it was formed contemporaneously 
with the strongly-marked monoclines occurring elsewhere in this general region, it is probable 
that these are not the original slopes. Later faulting and regional warping may have somewhat 
changed the original attitude of the fold, but observations by which the effects of these later 
movements might be distinguished are lacking. The extreme uniformity of the eastern slope 
precludes the existence of faults of great magnitude in that region. The maximum difference 
in the slope measurements (70 would cause a change in elevation between Flagstaff and 
Little Colorado River of but 350 feet, or about the thickness of the Kaibab limestone. This 
jBgure roughly represents the greatest possible displacement by faulting that would be likely to 
escape detection in the area considered. A fault of greater throw would cause either a notice- 
able break in the continuity of the surface or would bring other formations than the Kaibab 
limestone to the surface, and neither of these features was observed. 

The Coconino fold, which bounds the Coconino Plateau, lies mostly outside of the area 
covered by this report, but it may be briefly described. This fold is, for the most part, an 
asymmetric open anticline. The dip of the southwestern limb (1 J®) is, however, so much less 
than that of the northeastern limb (20°) and is so nearly parallel to the dip of the strata farther 
northeast that the fold assumes the character of a monoclinal flexure, as this term has been 
applied to displacements in the Plateau country. This may be seen by comparing the Coconino 
fold with the well-known Kaibab displacement, which adjoins it on the north; they have several 
features in common. 

The steep limb of the Coconino fold is clearly defined for 25 miles from the rim of the Grand 
Canyon near Hance's cabin to Coconino Point, in the Little Colorado Valley, at first trending 
southeast and then gradually swinging around to the east. Throughout this distance the dip 
of the flexure is 20® to the cast and north. The displacement produced by the flexure, meas- 
ured on the Kaibab limestone, which is the surface rock of the plateau, is 100 feet at the rim 
of the canyon, but gradually increases until it is over 1,000 feet at Coconino Point. At the 
latter locality the flexure turns southward for 3 miles and then southwestward, and the dip 
of the strata increases to 45°. A rather symmetric eastward-pitching anticline is thus formed 
which is strikingly displayed through the stripping of the soft strata overlying the Kaibab 
limestone. (See PI. II, A, p. 17.) The dip of the flexure (45° on the south side of Coconino 
Point) gradually decreases in a westward direction until in a distance of 15 miles it has flattened 
to 1}°. This is the dip of the southwestern limb of the Coconino anticline, and it prevails, with 
minor irregularities, throughout a belt of country some 15 miles wide, extending northwestward 
to the Grand Canyon in the vicinity of Bass's camp. 

The sharp dip seen in the strata at Hance's cabin, on the south rim of the Grand Canyon, 
may be traced northwestward for over 8 miles in the canyon, as far as Shoshone Point. The 
direction of the flexure makes it certain that it is a continuation of the monocline described 
by Dutton* as associated with the West Kaibab fault. The disappearance of the flexure 

1 op. cit., p. 128. 



GEOLOGIC STBUCTUBE, 85 

in the region north of the canyon, however, remains to be traced, as the structure is complicated 
by the monocline which crosses the Powell Plateau, 6 miles farther west. 

At Black Point, on the west side of Little Colorado River, 8 miles below Black Falls, is a 
previously undescribed fold of considerable magnitude, which will be called the Black Point 
monocline. It appears to be a true monocline, as the strata on opposite sides of it have the 
same strike and dip. It may be traced 'from Black Point S. 15° W. for 10 miles, as far as Doneys 
Cone, where it disappears under recent basaltic lavas. The dip of the strata is 15° S. in this 
part of its course, but at Black Point it decreases to 5° and in the region to the north it flattens 
to less than 1° E., the normal slope of the surface. 

The cherty Kaibab limestone forms the surface of the fold between Black Point and Doneys 
Cone and also covers a considerable area to the northwest, locally known as the "limestone 
mesa." Upon the back of the fold, resting on the cherty limestone, are striking isolated buttes 
of red shale. At Black Point, where the greatest thickness of folded strata has been preserved 
under a capping of basalt, the Kaibab limestone and the Shinarump group are involved in the 
displacement, but whether still younger formations are present was not determined. It is 
estimated that the vertical displacement that produced the monocline is not less than 800 feet 
at Black Point or more than 500 feet at Doneys Cone. The monocline appears, therefore, to 
flatten out to the west as it does north of Black Point, although at a gentler rate. On the 
whole, the Black Point monocline bears a strong resemblance to the Coconino fold, but the 
uplift that produced it was not so great, nor is it so distinctly marked topographically on accoimt 
of the smaller extent to which the red beds overlying the cherty limestone have been removed. 
It may be noted also that the dip of the Black Point monocline is in the opposite direction from 
that of the Coconino fold. 

The country between the Black Point monocline and Grand Falls, 20 miles farther south, 
is floored with the red shales and sandstones of the Moencopie formation, but at Grand Falls 
the Kaibab limestone again comes to the surface. This suggests that another, though smaller, 
fold may exist in this vicinity. It is not probable that the presence of the limestone is due to 
faulting. 

In the White Mesa, on the north side of Hull Wash opposite Cedar ranch, another small 
fold may be observed. The southeast face of the mesa is a cliff caused by a fault having a trend 
of N. 25° E. At the point where the Grand Canyon wagon road passes around the north end of 
the cliff the Kaibab limestone, of which the mesa is composed, strikes N. 60° W. and dips 10° N. 
To the southwest the strata, having risen 400 feet, regain a horizontal position, which' they 
maintain as far as the head of the wash 3 miles beyond, where they are buried under basaltic 
lavas. The fold was not traced northwestward, but appears to die out within a short distance, 
and south of Hull Wash no certain evidence of its existence was observed. The meager exposure 
of this fold makes it difficult to form a definite idea of its extent or magnitude. It seems likely 
from the location that it should be considered as associated with the strong Coconino fold to 
the east. 

The Echo Cliffs monocline, east of the Kaibab Plateau, has been considered probably to 
continue southward nearly to Winslow, about 100 miles distant. A view of the Little Colorado 
Valley northeast of Coconino Point from a distance of 10 miles suggests another explanation. 
The Echo Cliffs topographic map shows that the country opposite the mouth of Moencopie Wash 
has a slope of 3° SE. The strata of the Shinarump group have a corresponding dip and can be 
traced in a southwesterly direction across the Little Colorado to the foot of Coconino Point. 
On account of the close dependence of the topography on geologic structure in this portion of the 
Plateau, this relation is considered to indicate that the Echo Cliffs monocline changes its direc- 
tion from south to southwest near Moencopie Wash and loses its identity east of Coconino 
Point. If this supposition should prove correct, the gentle fold at Winslow should probably be 
correlated with the equally gentle San Franciscan anticline if any correlation is possible. 

The evidence from the Coconino and Black Point folds assigns to them only a post-Triassic 
age. Dutton^ found that the Echo Cliffs monocline, which he considered very nearly coeval 

1 Op. dt., pp. 191, 205. 



36 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

with the East Kaibab fold, involyed Eocene strata along the southern border of the High Plateaus 
of Utah. He concluded that East Kaibab fold was formed in Pliocene time, shortly before the 
period in which the Grand Canyon originated. Davis,* however, has pointed out that the great 
retreat of the Vemulion ClifiPs around the north end of the Kaibab Plateau since its uplift, as 
compared with the slight retreat of the upper walls of the Grand Canyon since the beginning of 
the canyon cycle, clearly indicates a greater age for the Kaibab and Echo Cliffs displacements. 
This view is supported by the observation of Walcott' as to the unfractured condition of the 
beds of the Aubrey group in the northern part of the East Kaibab monocline, from which he 
inferred a considerable thickness of overlying sediments at the time the fold was formed. It is 
probable that these folds originated during the Eocene-Miocene interval. 

FAULTS. 

None of the major faults of the northern part of the Grand Canyon district cross the San 
Franciscan volcanic field, although the fault which forms the western boundary of the field may 
prove on further study to be a southward prolongation of the Hurricane fault, offset to the east. 
It is not improbably however, that the region is intersected by a considerable network of minor 
faults. A few such faults were observed, but no particular search was made for them, as they 
are difficult to detect. The older faults — those antedating the eruption of the basalt of the first 
period of volcanic activity — are the more difficult to locate because the relief produced by them 
was effaced during the peneplanation of the region. The younger faults, formed after the erup- 
tion of the basalt of the first period, but before the eruption of the basalt of the third period, 
may be successfully located in the area outside of the recent basalts, for the relief produced by 
them still remains. Within the area of the recent basalts they are hard to locate, although in 
a few places such features as the linear arrangement of the small basaltic cones may give a clue 
to their situation. 

Only six faults were actually observed, but the presence of a number of others was recog- 
nized, especially near Oak Canyon. They are all located in the area of the older basalt or 
outside of the lava field. Five of the faults range in direction from N. 15° W. to N. 25° E., and 
the sixth trends N. 65° W., which is approximately the strike of several small faults east of Oak 
Canyon. As these were the only two general directions noted, the minor faults may prove to be 
grouped into two sets striking broadly north-south and east-west. 

Dutton first called attention to the fact that practically all the major faults of the region 
north of the Grand Canyon have lowered the country on their west sides. The same is true of 
the fault which has given rise to the Aubrey Cliffs west of Williams. But of the five minor 
faults observed, including the Bright Angel fault at the Grand Canyon, four have lowered the 
region on their east sides. 

The age of the faults can be fixed in relation to the periods of volcanic activity. They are 
all younger than the first and older than the third or last general period of eruption. (See p. 95.) 
This means that the faulting occurred after the great erosion of the Grand Canyon district had 
been completed and refers it to the canyon cycle. The faults are therefore of post-Pliocene age. 

The faults are here described in order of magnitude. The scale of the general geologic map 
(PL III) is so small that it is impracticable to indicate them on it 

The Aubrey Cliffs fault, west of Williams, where the wagon road to Ash Fork crosses it, has 
an average direction of N. 15° W. It may be traced in the escarpment to which it has given 
rise for over 10 miles southward, and it dies out in about the same distance to the north. The 
total throw is at least 1,000 feet to the west. The displacement is distributed between two 
faults, the eastern one having a throw of 250 feet, the western taking the remainder. In the 
face of the uplifted block near the top is exposed the Kaibab limestone, overlain by basalt of 
the first period of eruption, and the same lava is found on the dropped block to the west. There 

1 Op. cit., pp. 140-141. 

* Waloott, C. D., Study of a line of displacement in the Grand Canyon of the Colorado in northern Arisona: Bull. Oeol. Soc. America, vol. 1, 
1889, p. 49. 



GEOLOGIC STBUCTUBB. 37 

has practically been no retreat of the strata to the east from the fault line, and this indicates 
the recency of the movement. 

The Oak Canyon fault may be traced from Woody Mountain, 7 miles west-southwest of 
Flagstaff, for 16 miles along the course of the canyon to the Verde Valley, and is continued still 
farther south. Its trend is very straight and is true north and south. In the vicinity of Woody 
Mountain the fault has a throw of 600 feet to the east and is marked by a steep cliff. North 
of the mountain the cliff is not seen, and the fault apparently dies out before reaching the rail- 
road, at what point can pot be said, as the area is covered by more recent lava flows. As in the 
case of the Aubrey Cliffs fault, the uplifted block has been little dissected since the faulting. 

The fault that produced the White Mesa, north of Cedar Ranch, strikes N. 25° E. and has a 
throw of not less than 400 feet to the east. From the south end, where it is obscured by more 
recent lavas, it was traced northward for 5 miles, but it has a much greater extension in that 
direction. The face of the mesa is slightly eroded and is cut by several small ravines; the 
fault line at its base is hidden by recent flows of basalt. 

The fault on the north side of Clark Valley strikes N. 65® W., has a throw of 300 feet to 
the south, and in this locality marks the boundary of Anderson Mesa. The fault line is easily 
traceable on account of the difference in level of the Kaibab limestone and capping basalt on 
the two sides of the valley from Greenlaw's sawmill to the Ice Caves, beyond which it was not 
studied. The uplifted block appears to have suffered little erosion, but in the dropped block a 
shallow valley has been cut, which narrows somewhat and deepens at its north end, forming the 
head of Walnut Canyon. 

Although it was not closely observed, there appears to be a fault of perhaps 300 feet throw 
to the east along the base of the Black Point monocline in the vicinity of Doneys Cone. It 
has the same direction as the fold, N. 15® E., but is probably of later age. 

South of Coconino Point, near Hull Wash, are several faults, or a single fault of irregular 
course, with a throw to the west, that have brought the red shales of the Moencopie formation 
into contact with the cherty Kaibab limestone. This would indicate a throw of 300 to 400 feet. 
Where first observed near Hull Wash the direction of the fault was N. 45® E. 



CHAPTER III. 

GEOLOGY OF THE VOLCANOES AND LAVA FIELDS. 

OUTUNE OF CHIEF EVENTS. 

This chapter is devoted to the geologic phenomena of the volcanic region proper as they 
are displayed in the field. The igneous rock names are mostly simplified forms of those given 
in Chapter V, which have been based on characters determined in the laboratory. Brief descrip- 
tions of the rocks and the amounts of the principal chemical constituents are given for the 
general reader who may not be interested ia the detailed petrographic descriptions found in 
Chapter V. The sununation of the partial analyses is generally low; the diflFerence between it 
and 100 per cent (more or less) indicates the amount of water, phosphoric pentoxide, and other 
minor oxides which are omitted. 

As the volcanic phenomena are taken up, broadly speaking, in historical order, a brief 
outline of the chief events may be given. Three general periods of volcanic activity, separated 
by intervals of quiescence, occurred in the San Franciscan volcanic field. The phenomena of 
the first period were of a simple nature and consisted of widespread eruptions of basalt from 
small cones. During the second period various lavas, ranging from andesites to rhyolites, 
were erupted and built up a few large cones. This period was further marked by laccolithic 
and semilaccolithic intrusions contemporaneous with the volcanic extrusions. The third period 
closely resembled the first in that it witnessed the eruption of a single lava — a basalt — but it 
was characterized by the formation of a lai^er number of cones and a less widespread distri- 
bution of the lava. 

FIRST GENERAIi PERIOD OF ERUPTION. 

The first general period of volcanic activity is represented by a basalt which covers an area 
much larger than the San Franciscan volcanic field. The lava occurs predominantly as flows; 
fragmental material, except in cinder cones, was observed only at Marble Hill in the form of 
rather fine ash and represents an initial stage of activity. The original volume of the erupted 
lava, obtained by multiplying a total area of 3,000 square miles by an estimated average thick- 
ness of 50 feet, is calculated to be 30 cubic miles. Scattered lava and cinder cones mark the 
positions of the vents at which eruptions occurred. In general the cones do not exceed 
700 feet in height, although a few are over 1,000 feet. It is estimated that of the 300 basalt 
cones in the region 100 belong to the first period and 200 to the third period of eruption, A 
smaller proportion of cones is assigned to the first period because where only basalt of this 
period is present, as in the southwestern part of the field, cones are less numerous than in those 
areas where basalt of the third period occurs. Eruptions from fissures are indicated by the 
direct connection between dikes and flows in the walls of Oak and Sycamore canyons. Erosion 
has not been favorable, however, to the exposure of dikes, so that no idea can be formed as 
to the relative importance of this mode of eruption. 

The flows commonly have a thickness of 25 to 75 feet and at no observed point do they 
exceed 200 feet. Along the southern edge of Switzer Mesa, near Flagstaff, the thickness is 15 
feet, but to the north it appears to increase slightly. The general thinness of the flows seems 
unusual considering the large area they cover. It may be explained in part by the fact that 
most of the exposures are at the periphery of the field and consequently near the ends of the 
flows, the thicker central parts having been covered by more recent lavas. The flows, on the 
whole, do not have scoriaceous surfaces; normally they are dense throughout, and columnar 

38 



GEOLOGY OF VOLCANOES AND LAVA FIELDS. 39 

structure occurs in both the thm and the thick flows. Presumably the lava must have had a 
high degree of liquidity to spread out so thinly over so large an area. 

The flows are more or less disintegrated and their surfaces are covered by a mass of loose 
fragments embedded in a thin layer of soil. At some localities, however, considerable weather- 
ing of the lava has resulted in the formation of a residual clay.^ Such an area south of 
Flagstaff has been described by F. N. Guild,' who says: 

On the surface the country has the appearance of a rough lava bed with a minimum of soil. When this is removed , 
together with the fragments of lava, there is revealed a thin blanket deposit of pure red clay free from lava fragments 
and of a uniform thickness of about 22 inches. Below this deposit the material is similar but less thoroughly decom- 
posed into clayey constituents. The uniformity of the clay deposit has led me to conjecture that frosts have had much 
to do in its concentration by bringing the undecompoaed lava fragments to the surface. The presence of this deposit 
can be inferred in many places, where the loose lava fragments are not too numerous, by the deep cracks in the soil, 
some of which are an inch across and 1^ feet deep. 

The basalt is typically very dark gray in color on fresh surfaces. An aphanitic ground- 
masS; which occasionally might be considered microcrystalline, contains about 10 per cent of 
lusterless or iridescent olivine phenocrysts. The chemical composition is given by the follow- 
ing partial analysis: 

Partial analysis of basalt of first period of eruption. 



SiOa 47.7 

AlaO, 15.3 

FeA 5.9 

FeO 4.8 

MgO 7.3 

CaO 11.8 



NajO 2.5 

KaO 6 

COj 1.9 

TiOj 1.4 



99.2 



The olivine is generally much altered, but as the rock is otherwise entirely fresh the calcite 
present must be an infiltration product. 

The basalt rests on the Kaibab limestone in the western part of the field, except near 
Sycamore Creek, where the Shinarump group is present. In the area east of a line joining 
Anderson and Cedar Ranch mesas, approximately one-third of the region, the lava generally 
overlies the Shinarump group. The surface on which the lava rests is a peneplain, and observa- 
tions at numerous localities show that the eniptions of the first period occurred when the pene- 
plain was undissected." 

Where the basalt rests on the resistant Kaibab lunestone erosion has been confined mostly 
to the cutting of canyons and has resulted in the removal of a minimum amount of lava. Where 
the lava rests on the soft strata of the Shinarump group it has been much reduced in area by 
undermining and now caps only the mesas and buttes which rise 200 to 1,000 feet above the 
surrounding country. This is well illustrated along the eastern boundary of the volcanic field, 
in the Little Colorado Valley, and at Cedar Ranch. It is strikingly displayed along the entire 
east side of the Black Mesa and may be seen on a smaller scale near Flagstaff, where the lava 
caps a mesa whose top is 50 to 200 feet above the Kaibab limestone, depending on the thickness 
of the intervening Moencopie formation. 

It is estimated that the basalt originally covered 3,000 square miles in the San Franciscan 
volcanic field. As it now covers 2,200 square miles its area has been reduced more than one- 
fourth. The volume of lava removed is probably not more than one-fifth of the total, because 
erosion has been confined largely to the periphery of the field, where the flows are thinnest. 

Similar basalts resting on a peneplain are widely distributed in the surrounding plateau 
country. They extend unbrokenly west and south of the area included within the San Fran- 
ciscan field. The extent of the basalt to the west is unknown, but to the south, in the Black 
and Mogollon mesas, it covers more than 1,000 square miles. A similar lava also occurs in 
the Black Hills, on the west side of the Verde Valley opposite the Black Mesa. East of the 

1 For analyses of the lava and clay, see p. 150. 

* Personal communication. 

* Robinson, H. H., The Tertiary peneplain of the Plateau district and adjacent country in Arizona and New Mexico: Am. Jour. Scl., 4th aer. 
vol. 24, 1907, pp. 109-129. 



40 SAN FRANCISCAN VOLCANIC FIELD, ABIZONA. 

San Franciscan region the nearest area is in the Rabbit Ear Mountains, 80 imles distant. This 
is an isolated field which once covered between 500 and 1,000 square miles but is now eroded 
into many mesas and buttes, some of the latter being volcanic necks. Red Butte, north of 
the San Franciscan region, as well as Mount Trumbull and the other higher mesas near it, 
north of the Grand Canyon, are capped by the older basalt. Button's description ^ of the 
basalts of the Uinkaret Plateau may be correctly applied to lai^e areas in the southern part 
of the Plateau country. At all these places the basalt rests on a peneplain. Although these 
localities can not be exactly correlated, it is believed that all of them, with the possible exception 
of a single area in the Little Colorado VaUey, were peneplained during the same cycle of erosion. 
It thus seems probable that the basalt resting on a peneplain at the localities cited and even 
over a wider area in the southern plateau country was erupted during a period that essentially 
coincided with the first general period of eruption in the San Franciscan volcanic field. 

SECOND GENERAL PERIOD OF ERUPTION. 

During the second period of volcanism six isolated cones of lai^e size and a somewhat 
greater number of small cones were formed by the eruption of lavas ranging in composition 
from andesites to rhyolites. These cones are individually described under the heading *' Volca- 
noes of the second period.'' Several laccolithic and semilaccolithic masses are also assigned 
to this period, as they are directly correlated with the eruptions by the upturned lavas on 
their flanks. Of these masses Marble Hill is a true laccolith, but Elden Mountain and probably 
Slate Mountain present a unique combination of volcanic extrusion and laccolithic intrusion 
in the same geologic unit. They are separately described under the heading ^'Laccoliths of 
the second period. " 

VOLCANOES OF THE SECOND PEEIOD. 

SAN FRANCISCO MOUNTAIN. 
TOPOOSAPHY. 

The dominant feature of the region is San Francisco Mountain (see PL V), which rises 
in San Francisco Peak to a height of 12,611 feet above sea level, or about 5,000 feet above the 
surface of the plateau. It is by far the most prominent landmark in this portion of the Colo- 
rado Plateau and is, indeed, the highest elevation in the Southwestern States. 

The outline of the mountain, seen from any point sufliciently distant to mask the 
topographic details, conveys a strong impression of the volcanic origin of the mass. The 
slightly concave slopes rise uniformly on either side and the rather irregular crest line indicates 
that the cone has been somewhat eroded. On closer approach the outlying cones and lavas 
confirm the impression received at a distance. The slopes still retain their regular outline 
but are scored by ravines and on the east and southeast are interrupted by secondary cones. 
The crest line is resolved into six principal and several minor peaks, all over 11, 000 feet in eleva- 
tion, separated by divides of but little lower altitude. Three of these peaks have received 
names. On the north is the highest, San Francisco Peak; on the south are Agassiz and Fremont 
peaks, with elevations of 12,340 and 11,940 feet, respectively. In the view from Flagstaff, at 
the southern base of the mountain, where the eastern slope is hidden by intervening hills, the 
long, gently concave western slope at once reveals the character of the mountain. Likewise 
in views from the east, which exhibit the maximum amount of erosion, there is no diflSculty 
in perceiving the nature of the mass; in fact, when the details of the geology are known, this 
viewpoint has a greater interest than any other, as will appear later. 

The inclination of the outer slopes of the mountain about the rim, which closely agrees 
with the dip of the lava flows, is between 20° and 22° and decreases uniformly toward the 
foot slopes. Thus the surface of the flows at the northeast base of the cone has an inclination 

> Op. citv pp. 104-112. 




si- 



GENERAL GEOL( 

R. B. Marshall, Chief Geographet 
T. G. Gordmo. Geogrspher m charge 
Topography by Paaison Chapman, 1. F. Slaug 

Control by H, L. Baldwin, Jr., J. T. Stewart, 
andT. A. Green 

Surveyad in 1907 and 190B 



U. 8. QEOLOQICAL SURVEY 



PROFESSIONAL PAPER 76 PLATE VI 




£000' above sea levd 




2 
3 

4 



1 1 
1 1 

1 r- 
i I 
1 1 





- ... . ,^ , X on It ^.ra period of eruption); 7, rhyolite (second general period of 

1, A uvium (Quaternary); 2. Pennsylvanianformatia. ** J[ \ ' ^ /^# u«,ki^ u:ii. i^ hi^cait 

. \ o u ui _i J •/ / J Jve portion); 12, granite porphyry of Marble Hill; li, basalt 

eruption); 8, hornblende dacite (second generar ^ /» ' s r r f j 



(first general period of eruption). 



SAN FRANCISCO MOUNTAIN. 



41 



of 5°, whereas that of the flow oast of Fort Valley is but 2°. The maximum observed slope, 22°, 
is on the average about a mile, measured along the slope, below the former summit of the cone. 
(See cross sections, PI. VI.) A slope of 33° at the original summit is indicated, however, by 
the restored outline. For comparison with San Francisco Mountain in respect to form. Mount 
Shasta, in California, has been chosen, and the outlines of the two volcanoes are shown in fig- 
ure 4. In this figure the cross section of Shasta is drawn in an east-west direction through 
the summit and Shastina, the outline of the main cone being indicated by the dotted line; the 
section of San Francsico Mountain is drawn along the line B-B' of Plate V. The platforms 
on which the cones rest have been brought into coincidence so that the relative size of the 
two volcanoes is correctly represented. The resemblance of the two is very close and perhaps 
the more striking because the restoration of San Francisco Mountain was made without reference 
to any other volcano. It would seem that so close a similarity in form could result only from 
similar factors of development, which is further indicated by the general resemblance of the 
several lavas of the two volcanoes and their conmion mode of eruption. 

The upper slopes of the mountain down to elevations of 9,000 or 10,000 feet are scored by 
numerous ravines which are sharply incised and terminate abruptly at the waste fans that 
have been built out from their mouths. The most striking erosional feature o^ the mountain 
is the large interior valley which heads under San Francisco and Agassiz peaks and runs north- 
eastward for 3 miles, completely breaching the eastern wall of the volcano down to the level 
of 8,500 feet. The .head of the valley is divided into two arms by a prominent ridge which 






1- 






2 



3 Miles 



FiouBE 4.— Profiles of Mount Shasta and San Francisco Mountain. 

starts at the western crest of the cone, nearly midway between the two peaks above mentioned, 
runs eastward at an average elevation of 11,250 feet for three-quarters of a mile, and thence 
drops sharply to the valley floor. The eastern extension of the valley was obstructed by 
Sugarloaf Hill, at the foot of which it turns abruptly northward and is continued in a box 
canyon for 2 miles to Deadman Flat. This valley gives to the crest of the mountain the shape 
of a horseshoe, the open end being toward the northeast. 

The distance separating the ravines on the north, south, and west outer slopes of the cone, 
where they have developed on lava of the same stage of eruption, ranges between 0.5 and 0.7 
mile. Such uniform spacing seems rather to represent the spontaneous adjustment of 
drainage to slope than to be the result of chance location of drainage lines along depressions. 
There are, however, several ravines whose position has been fixed by contacts between lavas of 
different stages of eruption. Such is the origin of the ravines on the east side of the moimtain 
and of those on the southwest side under Agassiz Peak. The position of the interior vaUey 
appears to have been determined, at least in part, by the fact that the northeast slope is more 
susceptible to erosive forces than the others. It is the shadiest side of the moimtain, conse- 
quently it has a greater accumulation of snow during the winter and a larger run-off during 
the spring melting. It also probably receives the greater part of the rain in the summer season. 
The conditions that have fixed the line of maximum erosion on the northeast slope of San 
Francisco Moimtain are not local, for Mount Taylor in New Mexico exhibits a similar arrange- 
ment of valleys, and Kendrick and Sitgreaves peaks do so to a less extent. 



42 BAN FRANCISCAN VOLCANIC FIELD, ABIZONA. 

The outer slopes above timber line, approximately 11,500 feet, are largely covered with 
angular blocks of lava which in this situation indicate pronounced frost action. The stripping 
by this process is most evident on the outer slope of San Francisco Peak and to a less extent on 
^assiz Peak; it is greater where the surfaces rather than the edges of lava flows are exposed. 
Landslides are not uncomon in the upper (western) part of the interior valley, where the 
surrounding walls, 1,500 feet high, are composed largely of ash and breccia. They are not 
present in the lower part, where the walls have decreased in height and are composed of lava 
flows. 

The material eroded from the upper part of the mountain and brought down the ravines 
has been spread over the lower slopes in a series of coalescing fans. These alluvial fans, as in 
most other mountains in the Southwest, constitute a prominent feature in the topography of 
the volcano, their long, gentle slopes and comparatively smooth surfaces contrasting sharply 
with the steep and rugged slopes above them (PI. VII, JS). The larger fans on the south and 
west sides of the mountain almost entirely cover the original slopes up to about 10,000 feet, 
whereas on the north and east sides the upper limit of fans does not exceed 8,000 feet. They 
encircle the base of the cone for two-thirds of its circumference, and the large area covered by 
them may be seen on the geologic map (PI. V). 'In contrast to the outer slopes, the floor of 
the interior valley between 9,200 and 10,500 feet is composed of morainic material deposited 
by a former glacier, and an outwash plain* extends from the terminal moraine to the east end of 
the valley. The alluvial fans and glacial material are now being dissected and the eroded waste 
is being redeposited on lower and flatter slopes. An estimate of the amount of alluvial material 
on the slopes and about the base of the cone shows that.it constitutes 75 per cent of the total 
quantity of material eroded from the mountain. This clearly indicates that on the whole the 
rate of disintegration has exceeded that of transportation. 

The amount of material Amoved from the slopes of the cone has not been large, and except 
along the ravines and above timber line the outer slopes may be considered as little altered by 
erosion. It is true that they are somewhat weathered and covered with a thin mantle of soil, 
which is prevented from washing by a heavy growth of spruce and fir, but on the whole the af ea 
of unconsumed interravine slopes is extensive. This condition shows that the mountain is in a 
youthfid stage of dissection. 

OSITEHAL BTBUCTTrRX OF CONE. 

The structure of the volcano may be clearly made out from the position of the lavas and 
breccias, which have a maximum exposure of 2,000 feet, in the walls of the interior valley, and 
also by the lava flows on the outer slopes of the cone. These beds without exception dip away 
on all sides from a central mass of igneous rock which forms the sharp ridge at the head of the 
interior valley. This ridge, therefore, must represent the neck of the volcano, and it gives 
the restored original outline of the cone (fig. 5 and cross sections in PI. VI) a high degree of 
accuracy. It happens that in the view of the moimtain from the east (PI. VII, A) the core 
ridge can be seen at the head of the interior valley, so that the original form of the volcano may 
be readily visualized in the field. 

EBUPTIVE HISTORY. 

I 

VARIETIES OP LAVA. ' 

San Francisco Mountain is formed by lavas and breccias belonging to five distinct stages of 
eruption. It is composite both with respect to the character of its materials and the varieties 
of its lavas. The main vents from which the lavas escaped, with one exception, are all located 
close together within the area occupied by tl\e core ridge at the head of the interior valley, and 
for this reas6n the volcano has a very symmetrical outUne. (See fig. 5.) 

Partial analyses of the lavas are given on the next page. 



PAPER T« PLATE Vlir 




A. INNER (SOUTHEAST) SLOPE OF SAN FRANCISCO PEAK, FROM CORE RIDGE. 

!t slope is composed of andesitic la«as and breccias of the first stage of activity. Middle slope, dacil' 
econd stage; basal flow, marked by cliff, escaped throug;h a depression in older rim. Uppei slope : 
ummit. darker colored than the dacite. is augite andesite of fi4h stage. Light-colored V-shaped i 
It right is the dacite of the third stage of activity. 




B. NECK OF AUGITE ANDESITE OF FIFTH STAGE OF ACTIVITY. 

Showing rude vertical columnar structure. 



SAN FRANCISCO MOUNTAIN. 45 

At several points in the walls of the interior valley, of which the most prominent is on the 
inner slope of San Francisco Peak (PI. VIII, A), the dacite fills depressions in the latite. The 
form of these depressions shows that they are ravines eroded on the slope of the latite cone 
after it became extinct. Evidently therefore an interval, of quiescence ensued between the 
close of the first and the beginning of the second stage of activity. It is difficult, however, to 
form a definite idea of the length of this period. These ravines are not as large as the present 
ones on the outer slopes of the cone. They are also cut, for the most part, in unconsolidated 
breccia and ash, whereas the present ravines are in lava. It would appear (assuming constant 
erosive forces) that the interval between the first and second stages was very much shorter 
than the time which has elapsed since the cone finally became extinct. 

A mass of dacite in the core ridge marks, in part at least, the position of the main vent of 
the second stage. The western contact with the breccia of the preceding stage is exposed at 
an elevation of 11,750 feet a quarter of a mile east of the San Francisco- Agassiz saddle. The 
contacts elsewhere are with younger rocks. The direction of several dacite dikes in the north 
wall of the interior valley indicates that the vent may have extended to the east end of the 
core ridge, where later rocks are now present. The size of the vent is approximately 1,000 by 
2,000 feet or more, depending on the position of the eastern boundary. This is at a horizon 
1,800 feet below the restored summit of the dacite cone. The structure of the core rock is 
diverse. In the western part of the vent the rock is compact and has a pronounced platy 
structure; it also has a very irregular vertical jointing system, but it does not develop columns. 



o 
u 






0^1 2 3 Miles 

Figure 7.— Section through San Francisco Peak, Peak A, and Schuls Peak, San Franciaco Mountain. 

Toward the east end it is more massive and tends to weather in spheroidal forms. The inter- 
mediate space is occupied by an agglomerate whose matrix is filled with large and small frag- 
ments of decomposed lava. The restoration of the cone shows that practically all the lava 
exposed in the walls of the interior valley, which represents by far the greater part erupted, 
came from this central vent. 

The position of the dacite of the second stage on the outer slopes of the mountain indicates 
that most if not all of it was erupted from lateral vents. This lava is, of course, younger than 
that which is exposed in the walls of the interior valley and which came from the central orifice. 
It appears, therefore, that the eruptions during the later part of the second stage came from 
small lateral vents, whereas during the earlier and longer part they came from a single central 
orifice. 

Schulz Peak, on the southeast slope of the mountain, is the largest of these secondary 
dacite cones and its relation to the main cone is shown by the section through the locality (fig. 7). 
According to this restoration the peak had an original height of 2,800 feet and has been lowered 
by erosion some 900 feet. These figures are based on the assumption that the cone was dis- 
tinctly conical rather than dome-shaped. The outer slopes are strongly scored by ravines and 
show little lava in place. The cone appears, on the whole, more dissected than San Francisco 
Mountain, perhaps because of its somewhat greater age and the less firm texture of the lava. 
An agglomerate, weathered in pinnacles, outcrops in one of the larger ravines on the south side 
facing Schulz Spring and locates with some precision the vent of the cone. 



46 SAN FRANCISCAN VOLCANIC FIELD^ ARIZONA. 

The large mass of dacite at the northeast base of the mountain clearly came from the 
small parasitic cone situated If miles north of Peak F at an elevation of 9;250 feet. The dacite 
capping Peak A also appears to have been erupted from a small independent vent. The lava 
has a dip of but 10^, compared with dips of 20^ to 30^ in the dacite at other localities of equal 
elevation, and the section through this locality (fig. 7) shows that it lies 600 feet above the 
restored slope of the main dacite cone. Of like origin are the flows on the southwest side of 
the mountain, and the vent from which they came is located on the outer slope below Agassiz 
Peak, at an elevation of 11,600 feet. These flows extend directly down the slope to an elevation 
of 9,000 feet, where they turn to the south. Their southernmost limit is on the north side of 
Fort Valley, at 7,500 feet. The ends of two other dacite flows are situated north of Leroux 
Spring at elevations of 8,000 and 8,500 feet. 

The dacite of the second stage is found at greater distances from the center of the cone 
than any of the other lavas because of the eruptions from the lateral vents. The surface slope 
of the flows at the northeast base of the mountain is rather flat — about 5° — ^whereas the side and 
front slopes are steep. At several localities on the cone, however, flows terminate on slopes as 
steep as 15^, and elsewhere, as at Elden Mountain and the Dry Lake Hills, dacite of the same 
composition forms craterless, dome-shaped cones characteristic of viscous eruptions. These 
facts and the generally slightly eroded condition of the flows lead to the conclusion that the 
dacite never extended much beyond its present limits. If lava flows only are considered, it is 
evident that San Francisco Mountain attained its maximum basal area during this second stage 
of activity. 

The dacite from both the central and the lateral vents forms thick flows of massive appear- 
ance and tends to weather in spheroidal forms. A characteristic feature of the lava in San 
Francisco Mountain, as well as at several other localities, is the presence of numerous small 
inclusions of a dark-colored igneous rock. These inclusions are segregation products more 
basic in composition than the dacite and they invariably contain hornblende, whether or not 
this is the characteristic dark mineral of the lava. 

The second stage of eruption appears to be divisible into two substages. During the first 
and longer one there were many eruptions of lava from a single central vent. They somewhat 
shattered the older latite cone, as shown by dikes now exposed in the walls of the interior valley, 
which radiate from the central core. . A single dike in the upturned sedimentary rocks on the 
flanks of the Marble Hill laccolith, at the northwest foot of the moxmtain, suggests that the 
Assuring was not confined to the immediate vicinity of the main conduit. The lava of the 
first eruptions overflowed the rim of the latite cone through several depressions and when 
these were filled up it overran the older rim throughout its circumference. Bestorations show 
that the volcano was built up by these eruptions to a height of 8,200 feet, or 800 feet higher 
than the latite cone of the preceding stage. Its volume was increased to 32 cubic miles by the 
addition of 11 cubic miles of material, whoUy in the form of lava. 

During the later part of the second stage the eruptions came from small lateral vents instead 
of from the central orifice. There were at least six such vents, all situated on the outer slopes 
of the cone. The eruptions from these vents were the last eruptions of the second stage, as the 
lava now forms the slopes of the mountain. They added nothing to the height of the cone, but 
increased its volume to 34 cubic miles by the addition of 2 cubic miles of material. It is prob- 
able that the mountain attained its maximum basal area at this time as the result of these 
eruptions of dacite from lateral vents. As during the first part of the stage, only lava in heavy 
flows was erupted. The entire stage, therefore, was characterized by quiet but powerful 
outwellings of lava in large volume. 

THIRD STAGE OF ERUPnON. 

The hornblende dacite of the third stage was observed only at a few localities in the upper 
part of the cone. It is probable that this lava occurs in small amount, estimated at half a 
cubic mile. The principal outcrop is just west of Peak D. It consists of a sheetlike mass of 



SAK FEANCISCO MOUNTAIN. 47 

lightr-colored lava, which cuts across and has slightly metamorphosed the dacite of the second 
stage of eruption and possibly the latite of the first stage. It may be recognized in Plate VIII, 
Ay as the light-colored V-shaped wedge (the lower part is talus) at the crest line of the mountain 
on the extreme right of the picture. On account of the eroded condition of the mass it is diffi- 
cult to determine its relation to the older dacite. The lava may lie in what was a rayine on 
the slope of the pyroxene dacite cone. A second locality is at the west base of Peak F, at an 
elevation of 11,500 feet, where the exposure consists of a thin bed of pumice. No lava of the 
third stage was definitely recognized about the foot slopes of the mountain; nearly all the 
dacite so situated clearly belongs to the preceding stage. It does not seem probable, in view 
of the meager outcrops in the upper part of the cone, that any flows of hornblende dacite reached 
the foot of the mountain. 

A confused mass of rock on the south-central side of the core ridge marks the position of 
the vent of the third stage. The rock is somewhat more compact than the efiFusive tj^pe; it is 
also more coarsely crystallized and contains a smaller proportion of phenocrysts. An irregular 
contact with the older dacite core occurs about halfway up the south slope of the ridge, and on 
the east the rock is decidedly bleached and altered along the contact with the later andesite 
core rock. On the south the rock is in contact with an agglomerate of the first stage. The size 
of the core is roughly 500 by 1,000 feet, but within this area are several masses of agglomerate 
which may not belong to this stage. 

Although the evidence is slight, it would appear that the eruptions of the third stage were 
in part quiet, in part explosive, and of small volume. Thej? were not as vigorous as those of 
the second stage, as shown by the smaller number of dikes and their closer confinement to the 
vicinity of the vent. The size of the conduit seems quite disproportionate to the amount of 
lava apparently erupted. This su^ests that the eruptive forces, although initially strong 
enough to open the vent, were too weak to expel lava from it. 

The field evidence is too indefinite for estimating the interval between the second and third 
stages of eruption. Tiie lavas of the two stages have the same chemical composition and only 
minor mineralogic differences. Their similarity points to a very brief interval between their 
eruption, as compared with the intervals between the other stages. The conclusion rests on 
the assumption that differentiation of the magma proceeded at a uniform rate, so that there 
is a direct relation between its extent and time. This is not true, however, for the entire sequence 
of lavas. The interval between the fourth and fifth stages, during which the maximum change 
in the composition of the lavas occurred, appears to have been no longer than those between 
the first and second or the second and fourth stages. 

FOURTH STAGE OF ERUPTION. 

The only outcrop of rhyolite in San Francisco Mountain is in the saddle between Agassiz 
and Fremont peaks. The lava occurs in thin flows composed mostly of light-reddish banded 
spherulitic varieties and a minor amoimt of lustrous black glass. It is evident that the original 
volume of the lava must have been small; it is estimated at half a cubic mile. In views from 
the south the mass may be distinguished from the underlying lavas by its lighter color. 

The core rock of this stage is found on the inner (northwest) slope of Fremont Peak at 
an elevation of 11,000 feet. Although none is in place, there is no doubt that it marks the 
position of the rhyolite vent. The rock is bluish-gray in color and thoroughly compact. Of 
the five core rocks it is the only one that may be seen megascopically to have a holocrystalline, 
or, to speak more strictly, a cryptocrystalline texture. The restoration of the volcano shows 
that the mouth of this orifice was originally on the outer slope, and the relation of the rhyolite 
to the dacite in the Agassiz-Fremont saddle indicates that the side of the cone was somewhat 
disrupted in the opening of the vent. 

The rhyolite eruptions are shown by field evidence to have occurred only between the 
second and fifth stages of activity. However, it is considered practically certain that the 
hornblende dacite was erupted immediately after the pyroxene dacite, so that the rhyolite 
must be assigned to the fourth stage of activity. 



48 SAN FBANGISGAN VOLCANIC FIELD, ABIZONA. 

FIFTH STAGE OF ERUPTION. 

The andesite of the fifth and last stage of eruption at San Francisco Mountain caps the 
summits of all the principal peaks and is the surface rock on the north and in part on the south, 
east, and west sides of the cone. At most localities it rests on the pyroxene dacite of the second 
stage, as is illustrated by Plate VIII, -4, where the dark-colored lava at the summit of the moun- 
tain is andesite and that below is dacite. In Peak F, however, it overlies the hornblende dacite 
pumice of the third stage, and the restoration of the cone shows that it bears the same relation 
to the rhyolite in the Agassiz-Fremont saddle. Eroded remnants of the andesite, veiy fine 
grained and rather scoriaceous, occur on the northeast (inner) slope of Peak A about 250 feet 
below the summit. From this vicinity to the base of the cone on the east side the andesite is 
in contact with the latite of the first stage, which came from a secondary vent. On the east 
side, also, the andesite is seen to overlie the rhyolite of Sugarloaf Hill. 

On the south a single stream of andesite extends 7i miles from the center (crater) of the 
cone, which is a greater distance than is covered by any other lava, so far as observable. The 
flows of this stage ]>robably extend to or beyond the base of the mountain on the north, east, 
and west sides, to judge from the thickness of the outcrops in upper slopes, although this can 
not be seen on account of the covering of later lava and alluvium. On the northeast side, 
however, the flows terminate on the middle slopes at a distance of 3 miles from the center of 
the cone. 

The mass of andesite at the east end of the core ridge (PI. VIII, B) marks the position of 
the orifice through which all the lava was erupted. On the west it abuts against older core 
rocks, and on the other sides, although the contact is covered by glacial drift, it would appear 
to be in juxtaposition with the latite (either as lava or core rock) of the first stage. The length 
of the plug east and west is approximately 1,500 feet and its width 1,000 feet. The rock exposed 
in the upper part of the core, now about 3,500 feet below the original summit of the cone of 
this stage, is dark gray in color, is imiformly compact, and has an aphanitic texture. Under 
the microscope it is seen to be more coarsely crystalline than the rhyolite core rock, which 
megascopically would be considered holocrystalline. Megascopically, therefore, the more 
coarsely crystalline andesite porphyry is defined as having an aphanitic texture and the less 
coarsely crystalline granite porphyry as having a holocrystalline texture. This anomaly in 
definition is due simply to the difference in the color of the two rocks and is but one of many 
encountered in the classification of rocks by their megascopic characters. 

Andesite dikes, so far as observed, are confined to the vicinity of the conduit through which 
the lava was erupted. One occurs in the core rock itself, indicating slight disturbances in the 
vent after the main eruptions had ceased. The presence of dikes only near the vent suggests 
that the outbreaks were not violent. 

The interval between the fourth and fifth stages appears to have been short. In fact, the 
regularity of the extensive contact between the pyroxene dacite and the andesite would seem 
to indicate that the interval between the second and fifth stages was no longer, perhaps, than 
that between the first and second stages. The bed of hornblende dacite pumice under Peak F 
may be considered as pointing to a short interval between the third and fifth stages. Pieces of 
the pumice an inch in diameter float in water about five minutes before sinking. The presence 
of such light material on a slope of 20° suggests a short period of time or the feeble action of 
erosive forces. In the absence, however, of any knowledge as to the relative erodibility of the 
different lavas, these estimates should not be given much weight. 

The fifth stage of eruption was characterized, then, by quiet but rather vigorous outwellings 
of andesite unaccompanied by explosive phases, as shown by the absence of any fragmental ma- 
terial. The lava was erupted in sufficient volume to overflow the entire circumference of the 
older rim of the volcano. However, the flows were not thick enough to bury the masses of 
pyroxene dacite and latite that escaped from lateral vents and rose above the slopes of the cone 
built up by the eruptions from the central vents. As shown by the restored outlines, the cone 



SAN FRANCISCO MOUNTAIN. 49 

reached its maxixnum height of 8,800 feet, or 600 feet above the summit of the dacite cone, at 
the close of this stage and its volume was increased to 38 cubic miles by the addition of 3 cubic 
miles of lava. 

V0LT7MS OF THE COHE AND OF THE UDIVID ITAL LAVAS. 

The volcano has been so slightly eroded since it became extinct that it is possible to restore 
the original outlines with considerable accuracy (see cross sections, PI. VI), and from them to 
calculate the volume. The volume obtained is the average of the volumes of the three cones of 
revolution generated by the restored outlines of the cross sections. The results are of course not 
exact, but the conditions of the problem do not appear to call for greater accuracy. Considera- 
ble differences will be noted in volumes of the cones generated by the three sections, which are 
due to the somewhat unsymmetrical form of the cone. Thase differences, however, will not 
affect the average result unless the cross sections have been inappropriately chosen. 

The calculations of the volume of the main cone and of the lava of each stage of eruption 
are as follows: 

Total volume of main cone of San Francisco Mountain. 

Cone generated by — Cubic miles. 

Section A-A' 33. 4 

Section B-B' 36. 8 

Section C-C' 38. 9 

Average nearest integer 36. 

Volume of latite cone (first stage) of San Francisco Mountain. 

Cone generated by — CuWc miles. 

Section A-A' 17. 8 

Section B-B' 20. 7 

Section C-C' 24. 5 

Average ,, 21. 

Volume of cone of San Francisco Mountain at close of second stage. 

Cone generated by — Cubic miles. 

Section A-A'' 29. 9 

Section B-B' 33. 6 

Section C-C' 36. 3 

Average 33.0 

The last figure includes the volumes of the hornblende dacite and rhyolite stages, estimated 
at 1 cubic mile. Corrected for this, the volume would be 32 cubic miles. 

The volume of dacite of the second stage in the main cone is the difference between 32 and 21 
cubic miles, or 11 cubic miles. To this must be added 2 cubic miles of lava erupted from lateral 
vents, so that the total volume is 13 cubic miles. 

The volume of andesite erupted during the last stage of activity is the difference between the 
total volume of the main cone (36 cubic miles) and tJie volume of the cone at the end of the 
fourth stage (33 cubic miles), or 3 cubic miles. 

The total volume of lava erupted from both the main and secondary vents is as follows: 

Total volume of lava erupted from San Francisco Mountain. 

Cubic miles. 

Latite (lava, breccia, and tuff) 21 

. Pyroxene dacite (lava) 13 

Hornblende dacite (lava) J 

Rhyolite (lava) i 

Andesite (lava) 3 

3S 
75008*— No. 76—13 i 



50 SAN FRAKCISGAN VOLCANIC FIELD, ABIZONA. 

PSOPORTIOV OF OOHE SRODZD SDTOS CSSSATIOIT OF VOLOAno ACTIVITY. 

Although the ravines and serrated crest line create the impression that the mountain has 
been considerably eroded, measurements do not bear out this idea. It is estimated from the 
restored cross sections that the present crest line is on the average 3,000 feet below the summit 
of the cone as it stood at the close of volcanic activity. This means that the height has been 
reduced over one-third, a very considerable amount. It must be remembered, however, that 
volumes are under consideration and that a large reduction in the height of a cone does not 
carry with it a corresponding loss in volume. 

The minimum amount of erosion that has occurred is shown in section A-A', through San 
Francisco and Fremont peaks; the maximum amount in section C--C', through the interior 
valley and the large ravine on the west side. An intermediate amount is exhibited by section 
B-B', through Agassiz Peak and Peak F. An inspection of the geologic map (PI. V) shows 
that sections similar to B-B' are the most common, and consequently that the amount of ero- 
sion of a cone generated by this section will approximately represent the average for the entire 
cone. It is of especial interest to know the proportion of the cone that has been eroded, because 
it gives a due to the actual age of the volcano and its relative age as compared with the other 
cones of the region. It also permits an idea to be formed of the accuracy of estimates of the 
erosion that have been based only on eye observations in the field. 

The least erosion, as stated above, is shown by section A-A'. It amounts to 4 per cent of 
the total volume of the cone generated by that cross section. For the cone corresponding to 
section B-B' the proportion eroded is 6 per cent, and the maximum amount of erosion for the cone 
generated by section C!-C' is equivalent to 12 per cent of the total volume. The amount of erosion 
for the whole cone has been obtained by averaging the above values weighted according to 
the proportion of the cone that has been correspondingly eroded. Thus, the value for the cone 
generated by section A-A' has been given a weight of 1, section B-B' 12, and section C-C' 5. 
On this basis San Francisco Mountain has lost 8 per cent of its total volume since volcanic 
activity became extinct. This final value does not include the erosion along a number of the 
ravines on the outer slopes of the mountain. A calculation shows, however, that the volume 
of this erosion is so small in proportion to the total erosion as to be negligible. That is to say, 
the ravines, with the exception of the interior valley and perhaps the one on the west side, 
are mere scratches on the surface of the volcano. 

A check may be had on the above calculation by considering the manner in which erosion 
has taken place. An inspection of the cross sections and the geologic map will show that 
erosion has tended to reduce the height of the cone without greatly altering the slopes below. 
The ravines do not extend more than halfway down the slopes of the mountain, the lower 
slopes being in general thoroughly protected from erosion by the heavy mantle of waste material. 
Under these conditions the portion of the cone that remains may be considered as a frustum 
having a height equal to one-half the height of the restored cone. Likewise the portion removed 
may be thought of as a frustum of equal height. Calculated in this manner the proportion of 
cone eroded is 6 per cent of the total volume. The amount of erosion according to the first 
method of calculation — 8 per cent — is one-third greater than this, so that it may be slightly in 
excess of the true value. 

It was formerly believed that the cone ha<l been much more severely eroded than is actually 
shown by this calculation. It was a surprise to find so small a figure for the actual volume of 
erosion. It may be instructive, therefore, to compare certain estimates of a qualitative nature 
that have been made regarding the degree of erosion of San Francisco Mountain with the true 
value in order to form an idea of their correctness. 

Dutton ^ has said that the cone **has long been extinct and is greatly battered by erosion.'' 
Salisbury ' says: '*San Francisco Mountain is another example of a volcanic mountain par- 
tially destroyed by erosion. The form of the old cone can be but imperfectly known.' ^ D. W. 
Johnson ' describes the mountain as follows: *'The once symmetrical form of the volcano has 

» op. cit., p. 120. 

« Salisbury, R. D., Physiography, 1907, p. 384. 

* A recent volcano In the San Franciaco Mountain region: Bull. Oeog. Soc. Philadelphia, vol. 5, No. 3, 1007. 



BAN FRANCISCO MOUNTAIN. 



51 



been so much dissected by streams and glaciers that we may regard it as in a more or less mature 
stage of its erosion history." The first two descriptions clearly imply extensive erosion. It 
may be a matter of doubt just how much erosion is necessary to produce a more or less mature 
stage of development of a volcanic cone having 20° slopes. Maturity of form, however, results 
from the entire consumption of the intervalley slopes, and in the case of San Francisco Moun- 
tain, with the ravines spaced as they are, this would be associated with extensive erosion. 
These descriptions exhibit a conmion tendency to greatly overestimate the erosion of the cone 
and indicate that ideas regarding the extent of erosion of this rhountain have been quite inac- 
curate when based simply on a general field view. 

It would not be surprising to find that the extent of erosion of volcanic cones, and possibly 
other mountain types, in arid and semiarid regions has been and is likely to be very generally 
overestimated. Dutton, for instance, applied the same description to Mount Taylor, in New 
Mexico, as he did to San Francisco Mountain. The cross sections of Mount Taylor, showing 
minimum and maximum erosion, are compared with the corresponding sections of San Fran- 
cisco Mountain in figure 8. A rough calculation places the amount of erosion of the Mount 
Taylor cone at 11 per cent of the total volume, which hardly warrants the expression ''greatly 
battered by erosion.'' This error is caused, 
perhaps, by the bare and rugged upper slopes 
being too strongly impressed upon the mind, 
whereas the lower uneroded slopes covered 
with alluvium are overlooked. Furthermore, 
not all the ravines of volcanoes, which play 
an important part in forming an idea of the 
extent of erosion, are due to erosion. They 
not uncommonly result from original diflFer- 
ences in the positions of separate flows. 

PI.ATFO&K OF THE VOLOAITO. 

The exposures on the flanks of Elden 
Mountain and Marble Hill show that at these 
two localities the platform on which San 
Francisco Mountain stands is the basalt-cov- 
ered surface of the plateau. The basalt is 
that of the first general period of eruption of 




Son Francisco Mtn 



Minimum erosion 




San Francisco Mtn 



Maximum erosion 



Mt. Taylor 



Minimum erosion 



Mt. Taylor 



Maximum erosion 






6 Miles 



the region and was somewhat eroded before figure S.— comparative erosion of Ban Fnmcisoo Mountain and Mount 

San Francisco Mountain was formed. Thus Taylor. 

it is probably more correct to say that the cone rests partly on the basalt, partly on the 
underlying sedimentary rocks. It is certain that the cherty Kaibab limestone (Pennsylvanian) 
lies beneath the mountain, and possibly Permian ( ?) and Triassic strata are also present. 
Broadly speaking, beds belonging to the Shinarump group lie east of a line from the north end 
of Anderson Mesa to Slate Mountain and the cherty limestone lies west of it, and this line 
passes beneath the volcano. But at the two nearest localities — Elden Mountain and Marble 
Hill — where sedimentary rocks are exposed, only cherty limestone is present, so that the 
strata beneath the volcano can not be precisely determined. A fissure located in the north 
wall of the interior valley of the mountain is filled with numerous pieces of greenstone, por- 
phyry, and siliceous sediments embedded in a soft fine-grained matrix of a light-brown color. 
The fragments of red sandstone which occur in the fissure may have come either from the 
Shinarump group or from the lower formations of the Aubrey group. Red sandstones that 
can not be distinguished megascopically in small specimens are found in all these formations. 
For the sake of simplicity the cone has been represented in the cross sections as resting upon 
the basalt, which in turn lies only upon the cherty limestone. 



52 



SAX FRANCISCAN VOLCANIC FIELD, ARIZONA* 



AOX OF THB YOLCAMO. 

The fact that but 8 per cent of the total volume of the cone has been eroded would appear 
to fix the cessation of volcanic activity at no very remote date. Activity must have begun 
s^>mewhat eariier in order to permit the eruption of such an extensive and well-differentiated 
Bi^rieH of lavas as occurs at San Francisco Mountain, but it is estimated that this interval was 
shorter than that which has elapsed since the cone became extinct. The mountain is younger 
than the basalt of the first general period of eruption, because its lavas overlie the basalt both at 
Elden Mountain and at Marble lUll. The lavas of San Francisco Mountain differ so notablv 
from the basalt in character, mode of eruption, and extent of erosion that they have been 
assigned to an entirely distinct period, which is called the second general period of eruption of 
the San Franciscan volcanic field. 



San Francisco Mountain is a composite volcano both with respect to the character of its 
material and the variety of its lavas. At the close of activity the summit rose 8,800 feet above 
the surface of the plateau and the base covered an area of about 110 square miles. Its total 
volume was 38 cubic miles. 

Erosion has now lowered the height of the cone on the average 3,000 feet, so that no trace 
of the former crater remains. A large valley has been excavated on the east side of the mountain 
for 3i miles from its center, completely breaching the wall of the cone down to the level of 8,500 
feet. This valley was occupied by a small glacier during a recent period, as is shown by the 
well-developed moraines and outwash plain. The outer slopes are scored by a number of 
smaller ravines, from the mouths of which alluvial material has been spread out in a series of 
coalescing fans. Both the ravines and the fans are noticeable features in the topography of the 
mountain. The fans border the base of the cone for over two-thirds of its circumference and 
on the south and west sides extend more than halfway up the slopes. Although the cone has 
the appearance of being considerably eroded, calculations show that but 8 per cent of the total 
volume has been removed since it became extinct. 

The structure of the volcano is clearly made out from the position of the beds exposed in the 
walls of the interior valley and on the outer slopes. They dip away on all sides from a central 
moss of igneous rock at the head of the interior valley, which is thus identified as the neck of the 
cone. The rocks here exposed show that all the principal vents were situated in close proximity 
to one another at this locality. Over 90 per cent of the erupted lavas came from these central 
vents, and this accounts for the symmetrical outline of the volcano. 

Five distinct stages of activity are recognized. They may be tabulated as follows: 

Stages of activity of San Francisco Mountain. 



Stage. 



1 
2 
3 
4 

5 



Type. 



Predominantly explosive; very active 

Quiet: vlRoroufl 

Predominantly quiet; weak 

Quiet; weak 

Quiet; rather vigorouii 



Lava. 



Volume 
of lava. 



Latite (lava, breccia, tuff) 
Pyroxene dacite (lava) . . . 
Hornblende dac^lte (lava). 

Rhyolite (lava) 

Andcsite (lava) 



Cubic 

miles. 

21 

13 



Height 
of cone. 



Feet. 
7,400 
8,200 
8,200 
8,200 
8,800 



The cone rests on the somewhat eroded basalt-covered surface of the plateau and so far as 
known lias not produced any pronounced change in the attitude of the strata beneath it. 

The lavas of San Francisco Mountain are younger than the basalt of the first general period 
of eruption, which caps the penej)lained surface of the plateau, as they overlie the basalt at 
Elden Mountain and Marble Hill. They are older than certain other basalts which overlie 
them a relation best seen in the vicinity of the schoolhouse on the Grand Canyon road west 
of the mountain. These basalts have been erupted from small cones and are elsewhere foimd 
nesting on the basalt of the first general period of eruption. The lavas of San Francisco Moun- 



KENDRICK PEAK. 



53 



tain therefore occupy a position between the two basalts in the volcanic sequence of the region 
and are assigned te a distinct period of activity — the second general period of eruption — on 
account of their marked difference in composition and mode of occurrence. 



KENDRICK PEAK. 



TOPOORAPHT. 



Kendrick Peak, the second largest cone of the region, is 11 miles northwest of San Francisco 
Mountain and rises 10,418 feet above sea level, or more than 3,000 feet above the surrounding 
country. The main mass of the mountain is distinctly symmetrical and has a characteristic 




Contour interval 200 feet 



^ >- 

PiouBE 9.— Topographic map of Kendrick Peak. A- A \ B-B', C- C\ Lines of sectiooa In fisure 10. Topography from map of Flagstaff 

quadrangle. United States Oeologlcal Survey. 

volcanic contour. The outline appears most regular when viewed from the south (PI. IX, A). 
It is more irreglar as seen from the north on account of a mesa-like tongue of lava and several, 
small basaltic cones. 

Since it became extinct the main cone has been reduced in height 1,000 feet. The slopes 
are somewhat dissected by erosion, which has resulted principally in the development of three 
large meridional ravines situated on the east, northwest, and south sides of the mountain. 
These ravines head near the geometric center of the cone and have lowered the height of the 
mountain vertically without shifting their divides. They are rather symmetrically spaced, the 
angular distances between them being 110°, 115°, and 135°. The heads of the ravines are 
separated by very narrow divides at an elevation of 10,250 feet, and from the extremity of the 



54 



SAN FBANCISCAK VOLCANIC FIELD, ARIZONA. 



I«N 



hi I 

^1 




C-Ti 



II 



00 



KVJ 



a 



Ji 

■7/ 



CO 



V-- 



CO 



cv 



I .'■' 



i I 



._iLL_ 







«>) 






s 



Oft 

£ 



western divide the summit of the moimtain rises 250 feet higher. On accoxmt of the obtuse 
intersection of the divides, the profile of the upper part of the cone (PI. IX, -4) is similar in 
appearance from all points of view and is thus a very characteristic feature in the topography 
of the volcano. The interravine slopes in general are undissected, except at high altitudes. 
^ -g The cone, as a whole, is in a youth- 

^1 g, g ful stage of its erosional history. 

The material eroded from the 
upper part of the mountain has 
S I been deposited about the base as 
S i alluvial fans or in inclosed basins 
I formed by intersecting basalt flows. 
„-| Bull Basin and the alluvial areas 
south and southeast of the moun- 
tain are examples of such basins. 
They are shallow, as wells sunk in 
them reach lava at depths of 10 to 
30 feet below the surface of the 
alluvial filling. Alluvium fills the 
ravines on the east and northwest 
sides of the mountain up to an 
elevation of over 9,500 feet and 
has a maximum surface slope of 
10°. Evidently it must have accu- 
mulated when the rate of disinte- 
I ^ gration was largely in excess of 
the rate of transportation. The 
fans are now undergoing dissection 
at the higher elevations and the 
eroded material is being deposited 
beyond the base of the cone; this 
duplicates the condition on San 
Francisco Mountain. 

The surface on which Kendrick 
Peak rests is not exposed at any 
locality within 6 miles of the cone, 
so that its nature must be judged 
from the general conditions that 
exist in the region. The mountain 
presumably stands on the some- 
what eroded basalt-covered surface 
of the plateau, as does San Fran- 
cisco Mountain. It is probable 
that the sedimentary rocks belong 
entirely to the cherty Kaibab lime- 
stone, as this is the youngest for- 
mation exposed at the localities 
nearest the cone. In the cross sections (fig. 10), for the sake of simplicity, the rhyolite is 
shown as lying directly on the limestone; this does not mean that the basalt is necessarily 
absent. 

EBUPTIVE HISTORY. 



« ft 

■I 



S.5 



11 

CO 
• • •■ 

•2 s 



s ^ 

■2 9 



8 



5 S 
US 



O Pi 



O 



VARIETIES OF LAVA. 



Kendrick Peak is a composite cone formed of five lavas which represent an equal number 
of eruptive stages. It is a simple cone, however, with respect to the material that composes it, 
as it consists entirely of lava flows. The lavas in order of eruption are rhyolite, pyroxene 



KENDBICK PEAK. 



55 



dacite, hypersthene dacite, andesite, and basalt, the succession being without break from an 
acidic rock to one of basic composition. An idea of. the chemical composition and differentia- 
tion of the lavas may be obtained from the following partial analyses: 

Partial analyses of lavas from Kendrick Peak, 



SlOs. 
AUO, 
FetOi 
FeO. 
MgO. 
CaO. 
NaiO 
KiO. 
TlOj. 



RhyoUte. 


Pyroxene 
aacite. 


Hvpers- 
thene 
dacite. 


Andeoite. 


68.8 

15.2 

2.7 

1.7 

. 4 

1.7 
4.4 
3.7 
0.3 


62.0 
• 17.0 
2.7 
2.7 
2.4 
4.0 
4.0 
3.1 
.9 


60.4 
17.0 
2.1 
3.9 
3.0 
4.4 
3.8 
3.0 
1.0 


56. 5 
16.3 
2.9 
5.1 
4.1 
6.1 
3.9 
2.2 
1.5 


99.2 


98.8 


98. 6 


98.6 



Basalt. 



47.4 

16.4 

3.4 

7.7 

8.6 

10.4 

2.5 

.8 

1.5 



96.7 



As the analyses show, the rhyolite, dacites, and andesite are slightly more basic than the 
corresponding lavas of San Francisco Mountain, which is due to regional differentiation. They 
have, however, the same megascopic characters as the lavas of the San Francisco cone. Even 
the rhyolite has typical spherulitic and glassy textures, although in chemical composition it 
more closely resembles the dacites than the rhyolite of San Francisco Mountain. It is inter- 
esting to note that the two dacites have practically the same chemical composition but differ 
slightly as to their dark minerals, thus duplicating the dacites of San Francisco Mountain. 

Basalt eruptions occurred at 12 or more points in the immediate-vicinity of Kendrick 
Peak, and a number of vents were well up on the slopes of the main cone. If observations 
were confined to Kendrick Peak this basalt would be included in the same period of activity 
as the more acidic lavas. Wider observations show, however, that it should be correlated 
with the basalts of the succeeding general period of eruption. It wiU not be further consid- 
ered in the description of the cone. 



FIRST STAGE OF ERUPTION. 



The rhyoUte erupted during the first stage of activity forms over three-quarters of the 
main mass of Kendrick Peak and covers more than 9 square miles of country to the northwest. 
It is most extensively exposed in the main cone on the divide between the eastern and north- 
western ravines and about the head of the eastern ravine. The exposures consist mostly of 
light-grayish spherulitic and black glassy flows intimately interbedded. The thinner glassy 
layers are commonly broken up and the fragments included in the spherulitic lava, thus form- 
ing a flow breccia. At the outer end of the northeast divide the rhyoUte has an average strike 
of N. 70** W. and a dip of 15® to 35° N. At the same locality, just above the contact with the 
pyroxene dacite, is a thick flow of black, lustrous obsidian. Its strike is N. 35® W. and its dip 
35® W. Near the contact with the dacite at the inner end of the southeast divide the average 
strike of the rhyolite is N. 65® E. and the dip 25® to 35® N. The exposures between these two 
locaUties have highly variant attitudes, the strikes ranging from N. 25® E. to N. 80® W., the 
dips from 15® to vertical. Smaller exposures of rhyolite in the main cone occur at three points 
on the north slope and at one point on the south slope. The lava at these localities is somewhat 
more compact than that on the divides and the dips indicate that it came from a centrally 
located orifice. At the locality on the south slope the rock has a dense aphanitic texture and in 
thin section is seen to be holocrystalline; it has evidently been recrystallized in the presence of 
heated water or vapor. 

The strike of the rhyolite at the outer end of the northeast divide and the inner end of the 
southeast divide shows that the lava was erupted from a vent situated near the point of inter- 
section of these divides. The directions of the flows at other points also intersect at about this 
same locaUty, which is at the center of the cone. The dip of the lava at the points oh the 
divides above mentioned is toward the center of the cone, so that the exposures must be in the 



56 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

rim of a crater. The outcrops between the two divides fall within the area of the vent thus 
indicated, which accounts for their highly diverse strikes and dips. The exact size and shape 
of the orifice can not be determined owing to the covering of soil, but the diameter can not be 
much over a quarter of a mile. 

The rhyolite northwest of the mountain forms a mesa-like mass which rises 260 feet above 
the surrounding basalt. The actual thickness of the lava, however, must be niearly 1 ,000 feet, 
if the surface on which it rests has been correctly located. The restoration of Kendrick Peak 
shows that, although the lowest flows of this locaUty may have come from that cone, by far the 
greater part of the lava was erupted from two independent cones of smallei: size. 

One of these cones is situated at the northwest foot of Kendrick Peak. Its south side is 
partly covered by later dacite flows, a fact which proves that the eruptions at this vent were 
contemporaneous with those of the first stage of the main cone. The rim is continuous on the 
south and west sides but is lacking on the north. The lava which issued from this orifice covers 
about 2 square miles southwest of the cone. It differs from the rhyolite elsewhere on Kendrick 
Peak in its lighter color — a deUcate pearl-gray — and its more pumiceous structure. The light 
color of the lava makes this cone and its associated flows conspicuous in views of Kendrick Peak 
from the west. 

The existence of the second cone has to be inferred. A cone is supposed to have been 
situated on the site of Bull Basin because of the circular form of that depression and the stnic- 
ture of the ridge bounding it on the northwest. (See figs. 9 and 10.) This ridge is composed 
of a light lilac-colored rhyolite, containing phenocrysts of light-brown biotite, exactly similar 
to the lava forming the mesa to the northwest. The strike of the lava conforms to the curva- 
ture of the ridge and the dip is 10® to 20® NW. As may be seen from cross section A-A' (fig. 
10), it is quite impossible to restore Kendrick Peak on the assumption that the lava in this ridge 
came from the main vent, and moreover there is no rhyolite of this character found in the large 
cone. The reconstruction of the cone on the site of Bull Basin has thus been made as shown 
in section A-A'. The indicated form is purely conventional. It is probable that the original 
cone was lower and that its removal was due to forces of a catastrophic nature rather than 
simply to erosion. 

It Ls estimated from the restorations of the mountain that the main rhyolite cone was 
originally about 4,600 feet high and its volume 3.6 cubic miles. In addition 1.7 cubic miles of 
lava was erupted from the vents on the north, so that the total volume of rhyoUte was 5.3 cubic 
miles. As the total volume of all the lavas at this locaUty was 6.2 cubic miles, it will be seen 
that the Kendrick Peak eruptions consisted predominantly of rhyohte. 

SECOND STAGE OP ERUPTIOK. 

The pyroxene dacite of the second stage outcrops on the northeast and southeast divides, 
where it overhes the rhyolite. The greatest thickness — 500 feet — ^is on the southeast divide, so 
that the initial overfiow occurred at this point. This lava is also the surface rock of the north- 
west slope and extends to the base of the mountain. The flow abuts against the small rhyolite 
cone, which partly obstructed and changed its course. So far as the exposures go, it can be said 
only thatj the dacite overran the sides of the older rhyoUte cone on the east and northwest. It 
may have covered them on the south and west, but it did not do so on the north side. At that 
locality the hypersthene dacite of the succeeding stage rests directly on the rhyoUte. 

The vent from which the lava escaped can be located only in a general way, from the direc- 
tions of the flows, as just west of the rhyolite core. There are, however, no exposures at this 
locaHty by wMch its size and shape might be determined. 

The lava of this stage for the most part is compact and light to dark gray or brown in 
color. An aphanitic groundmass contains phenocrysts of plagioclase and pyroxene. At some 
points, especially on the nortliwest slope, small segregations containing hornblende occur, which 
are of interest as suggesting the origin of the dark-colored basic inclusions in the dacites at 
other locahties in the region. 



KBNDBICK PEAK. 57 

THIRD STAGE OF ERUPTION. 

The hyperethene dacite of the third stage is the surface rock to a greater or less extent on 
all sides of the cone except the northwestern. There the pyroxene dacite of the preceding 
stage and possibly the rhyolite rose to a height which prevented its overflow. Where it is not 
covered by basalt, the lava is seen to extend to the base of the cone and it is probable that it 
extended farther beyond the base than the lavas of the preceding stages. The directions of 
the flows point to the location of the vent about a quarter of a mile northeast of the present 
summit of the mountain. The area was not visited, as it appeared to be quit« lacking in 
outcrops. 

The hypersthene dacite is typically dark gray in color, rarely almost black. An aphanitic 
groundmass contains numerous phenocrysts of feldspar and a few of pyroxene. This rock is 
generally distinguished from the dacite of the preceding stage by its darker color. However, 
as the color of the lavas is variant, the two may be confused in some hand specimens. 

The restoration of the cone shows that during this stage it reached its maximum height of 
4,800 feet and probably extended to its greatest basal area. The volume of lava erupted is 
estimated at 0.6 cubic mile, which gave the cone a total volume of 4.4 cubic miles. 

FOURTH STAGE OF ERUPTION. 

The andesite of the fourth and last stage of activity of Kendrick Peak occurs only as a few 
small flows at the summit and on the west slope. The vent from which it escaped was not 
located, but from the position of the flows it must have been near the vents of the preceding 
stages at the center of the cone. This lava is more compact and dense than the other lavas, 
and because of its greater resistance to erosion it now caps the summit of the mountain. 

V0LX7KX OF THB COHX AVD OF THB TBUmDVAL LAVAS. 

The total volume of the cone has been calculated from five restored cross sections, three 
of which are shown in figure 10, in the same manner as that of San Francisco Mountain. The 
results are as follows: 

Volume of cone of Kendrick Peak, 
Cone generated by — Cubic miles. 

Cro88 section A-A'' 4. 56 

CroflB section B-B^ 4.12 

Cross section C-C^ 4.72 

Cross section D-IV 4. 06 

Cross section E-E' 4.72 

Average 4. 5 

The differences in the individual volumes are of the same order of magnitude as those of 
San Francisco Mountain, and show that the two cones possess the same degree of synunetry. 

The volume of the rhyolite cone of the first stage of activity has been calculated from two 
cross sections with the following results: 

Volume of rhyolite cone of Kendrick Peak. 
Gone generated by — Cubic miles. 

Section A-A^ 3. 23 

Section B-B' 3. 54 

Average 3. 4 

If the erosion indicated by exposures on the east side of the rhyolite cone is taken into account, 
the above result may be increased by 5 per cent, which makes the volume of the original cone 
3.6 cubic miles, to which must be added 1.7 cubic miles of lava erupted from the secondary 
vents, so that the total volume of the rhvolite is 5.3 cubic miles. 

The total volume of the two dacites and the andesite must be 1.1 cubic miles, the difference 
between the final volume of the main cone (4.5 cubic miles) and that of the eroded rhyolite 



58 



SAN FKANCISCAN VOLCANIC FIELD, ABIZONA. 



cone (3.4 cubic miles). This has been apportioned among the three lavas on field evidence, 
without calculation, as follows: Pyroxene dacite, 0.4 cubic mile; hypersthene dacite, 0.6 cubic 
mile; andesite, 0.1 cubic mile. 

PROPOBTIOir OF con BRODED SIVCS CESaATZOir OF BRXTPTIOirS. 

The impression that Kendrick Peak is less eroded than San Francisco Mountain is probably 
created by the smaller scale on which the details of the topography are developed and by the 
absence of any striking erosional feature such as the interior valley of the larger mountain. 
Calculations show, however, that the same relative proportion of the total volume of both 
cones has been removed and strengthen the belief that estimates of such erosion are of doubtful 
value when based only on field views. 

The minimum amount of erosion is exhibited in section B-B' (fig. 10). It is 8.5 per cent 
of the total volume of the cone generated by that section. The maximum erosion is along the 
line of section C-C', and is equivalent to 20 per cent of the volume of the corresponding cone. 
An intermediate value of 6.5 per cent is obtained from section A-A'. An average value of 5 
per cent is obtained from four cross sections exclusive of section C-C'. From an inspection of 
the topographic map of the mountain (fig. 9), it is estimated that if the average value be given 
a weight of 4 and the maximum value a weight of 1, a fair average for the entire cone will be 
obtained. On this basis, then, the proportion of the main cone of Kendrick Peak that has 
been eroded since volcanic activity became extinct is 8 per cent of the total volume. 

AGE OF THE COITE. 

The only field evidence as to the relative age of the volcano is the fact that the acidic 
lavas underlie a basalt and are consequently older. From the fact that the cone has 
experienced the same proportionate erosion as San Francisco Mountain, and from the close 
similarity of their lavas and mode of eruption, it is certain that the two cones were formed at 
the same time. The lavas of Kendrick Peak are thus definitely placed in the second general 
period of eruption of the region. 

STTMICABT. 

Kendrick Peak consists entirely of lava flows erupted during four separate stages of activity. 
The cone attained a maximum height of 4,800 feet and covered an area of 40 to 50 square miles. 
The volume of the main cone was 4.5 cubic miles; including the lava from secondary vents, 
it was 6.2 cubic miles. 

Erosion has lowered the height of the cone about 1,000 feet and developed three large meri- 
dional ravines on the south, the east, and the northwest sides. At other localities the slopes in 
general are largely undissected. Alluvial fans cover the lower slopes and similar material 
fills a number of small basins about the foot of the mountain. The proportion of the main cone 
that has been eroded since it became extinct is 8 per cent of the total volume. 

The four stages of activity may be tabulated as follows: 

Stages of activity of Kendrick Peak. 



Stage. 



1 
2 
3 

4 



Typo. 



Active and vigorous 

Quiet, weak 

Quiet, weak 

Quiet, weak 



Lava. 



Rhyolite 

Pyroxene dacite 

Hypersthene dacite 
Andesite 



Volume. 



Cubic 

mile*. 

5.3 

.4 

.6 

.1 



Height 
of cone. 



Feet. 

4,400 

4,600 

4,800 

4,800 



The greatest activity occurred during the first stage, and, as shown by the volumes, the 
cone consists predominantly of rhyolite. The succeeding eruptions produced little change 
in the size of the cone, but the lavas form an interesting sequence in comparison with those of 
the other large volcanoes of the region. No fragmental material was observed at Kendrick 
Peak, and the eruptions throughout were probably quiet. 



BILL WILLIAMS MOUNTAIN. 



59 



The cone most probably rests upon eroded remnants of the basalt of the first general 
period of eruption and the cherty Kaibab limestone. 

The lavas of Kendrick Peak are older than the basalt of the third general period of erup- 
tion. They are placed in the second general period of eruption of the region on accoimt of 
their close similarity to the lavas of San Francisco Moimtain and the fact that the cone they 
built up has su£Pered the same proportionate erosion. 

BILL WILLIAMS MOUNTAIN. 
TOPOGRAPHT. 

Bill Williams Mountain lies 33 miles S. 75° W. of San Francisco Mountain and is the west- 
ernmost of the large cones included within the San Franciscan volcanic field. The mountain 




Basalt cone 
7400 



«> Basalt cone 
7300 



2 



BMiies 



Contour interval 250 feet 



FiousB ll.^Topographio map of Bill Williams Mountain. A- A', B-B', C-C', lines of sections in figure 12. 

Topograptiy by H. H. Robinson, 1903. 

rises about 9,100 feet above sea level or 2,400 feet above its foot slopes and if the lavas belonging 
to secondary vents are included, it constitutes the third largest center of eruption in the region. 
Bill Williams Moimtain is less symmetrical than the other large cones, except Mormon 
Moimtain, owing to the presence of secondary cones on the outer slopes and the manner in 
which the mountain has been eroded. It is not difficult, however, to recognize the volcanic 
origin of the mass, even in its present condition, for the general conical form is still preserved, 
as may be seen from the sketch map (fig. 11). The map emphasizes the symmetry of the cone 



60 



SAN FBANCISCAN VOLCANIC FIELD, ABIZONA. 



with respect to a central point; a feature that is not so evident in the field. In the view from 
Williams the moimtain presents a rather irregular outline, the summit rising behind several 
dome-shaped peaks of lower altitude. A more symmetrical outline is presented in views from 
the west (PI. IX, B), the several peaks then clearly appearing as the eroded remnants of a single 
large volcano, with a secondary cone 500 feet high on the lower northwest slope. 

The cone is somewhat dissected. The present summit is 800 feet and the crest line, as a 
whole, about 1,300 feet below the original summit. The southern rim is the most eroded and 
on the northwest and southwest sides the walls are cut by two large ravines that head at the 
center of the cone. The mouths of these ravines are well up on the slopes of the moimtain at 
an elevation of 7,500 feet, where the alluvial fans begin. The boimdaries between flows from 
secondary vents and those from the main orifice fix the position of several smaller ravines on the 
north and east sides of the cone, but erosion along these lines has been rather slight. The 
interravine slopes are essentially uneroded, showing that the mountain is in a youthful stage 
of dissection. 



SECTION A-A 




StCTION B-B 






SECTION C-C' 



L 




f^ /.\.^:rr rTfrm 



2 Miles 



FiotTKE 12.— Qeologic cross sections of Bill Williams Mountain. 1, Alluvium; 2, Carboniferous formations (undlflbrentlated); 3, basalt of third 
general period of eruption; 4, dadte of second general period of vuption; 5, andeslte of second general period of vuptlon; 6, basalt of flbrst 
general period of eruption. For lines of sections see figure 11. 

Alluvial fans cover the slopes on the south and west sides up to an elevation of 7,500 feet. 
The dissection of the fans on the south side is especially noticeable. Washes 10 to 25 feet 
deep cut the alluvium at an average space of 1 ,000 feet from its upper to its lower limit. The 
continuity of the alluvium with the lava slope above it has been broken in some places by 
lateral erosion at the heads of the washes, thus giving rise to what might be called hogbacks 
of alluvium. The washes are sharply intrenched and the interwash slopes are uneroded, a 
condition which clearly indicates a youthful stage of dissection. It is evident that although 
aggradation exceeded transportation when the material was laid down, at present the relation- 
ship is reversed. This is true at all the large cones. 

ERUPTIVE HISTORY. 
VARIETIES OP LAVA. 

Bill Williams Mountain is a composite cone formed by lavas and breccias of two distinct 
stages of eruption. The distribution of the material shows that about two-thirds of it escaped 
from centrally located vents and built a symmetrical cone, and that the remainder came from 



BILL WILLIAMS MOUNTAIN. 61 

secondary orifices situated either on the slopes or about the base of the volcano. The lavas 
in order of eruption are andesite and dacite. Their chemical composition may be judged by 
the following partial analyses: 

Partial analyses of lavas from Bill Williavis ^fountain. 



SiOs.. 

AlfOi. 

Fe»0,. 

FeO.. 

MgO. 

CaO.. 

NasO. 

XjO.. 



Andesite. 



Dacite. 



51.5 


66.0 


17.2 


16.1 


4.6 


2.3 


5.5 


1.8 


6.0 


1.5 


8.1 


3.6 


3.4 


4.7 


1.7 


2.9 



Both lavas correspond in their chemical and mineral characters, as well as in appearance, 
to the andesites and dacites that occur elsewhere in the region, and they unquestionably belong 
to the same general period of activity. The andesite is, however, somewhat more basic in com- 
position than the related lavas of San Francisco Moimtain and Kendrick Peak and in some 
respects resembles the basalt of the succeeding period. The dacite of Bill Williams Mountain 
and that of San Francisco Moimtain and vicinity are so nearly alike in megascopic appearance 
that they may be easily confused. 



FIRST STAGE OF ERUPTION. 



Andesite in the form of breccia outcrops at the heads of the ravines at the center of the 
moimtain and at the northern base east of the small dacite cone. In the form of lava flows it 
is found in the vicinity of Campbell's well and Bixler's ranch, on the south and west sides of 
the mountain. It may occur at other localities about the base of the cone, but the region was 
not examined in sufficient detail to reveal them. The attitude of the beds at the localities 
mentioned shows that most of the material came from a central vent which later was the main 
channel of escape for the dacite. The vent, to judge from the dacite plug, must have been 
approximately circular in form and somewhat over 500 feet in diameter. The elevation of 
the lava at Campbell's well (section B-B', fig. 12) suggests a secondary outbreak on the slope 
of the main cone, unless the restored outline of the andesite cone is much in error. All the 
observed exposures, as shown by the cross sections, represent material erupted during the last 
third of this stage of activity, and breccia is present in greater amoimt than lava. Explosive 
phases, therefore, exceeded in number those of a quiet nature, and it is inferred that the rela- 
tion prevailed throughout this initial stage of activity. The restoration of the cone indicates 
that it had an original height of about 2,100 feet and a volume of 0.8 cubic mile. 



SECOND STAGE OF ERUPTION. 



The dacite of the second stage outcrops in many beds on the inner slopes of the mountain 
and forms the greater part of the outer slopes. It also composes the group of small detached 
cones east of the main cone. At all these localities it is in the form of flows. Fragmental 
material (pumice) was observed only in the valley through which the lumber railroad runs, on 
the east side of the mountain. 

The position of the vent from which the flows outcropping on the inner slopes of the cone 
were erupted is fixed, half a mile west of the present summit of the mountain, by a circular plug of 
lava 800 feet in diameter. Erosion has removed the surrounding lava and breccia down to an 
elevation of 8,000 feet, and the core now rises with steep slopes 100 feet on the east side and over 
250 feet on the west side above the divide between the two interior ravines. It is clearly ex- 
pressed in the topography of the cone, as may be seen from the map or cross sections. The 
core rock does not differ markedly in appearance from that of the flows. It is, however, some- 
what more compact and in mass shows a definite vertical flow structure. 



62 SAN FBANCISCAN VOLCANIC FIELD, ABIZONA. 

Much of the lava which forms the outer slopes and therefore was erupted during the later 
part of the stage came from secondary orifices. The cones built by these eruptions are noticeably 
dome-shaped, and the flows form prominent ridges on the slopes. The irregular outline of the 
moimtain, as seen from WilliamS; is due to such eruptions, and the present summit of the moun- 
tain is formed by the lava from one of the secondary vents (section A-A', fig. 12). The posi- 
tions of three secondary vents, along a north-south line a quarter of a mile east of the central 
core, are marked by plugs. Small cones on the lower northwest and northeast slopes fix the 
positions of two other vents. Other secondary orifices are marked by the group of dome-shaped 
hills at the east base of the mountain. 

The dacite on the north and west sides extends 2 to 3 miles from the center of the cone. The 
steep fronts of these flows show that the lava never extended much beyond its present position. 
There is, however, one isolated remnant of dacite, about a mile north of Williams and just east of 
the railroad to the Orand Canyon, whose relation to lavas of Bill Williams Mountain was not 
determinable. The outcrop is entirely surrounded by basalt and rises 25 feet above it. The 
lava is dark gray in color, is compact, and contains very few phenocrysts; it does not especially 
resemble the dacites of Bill Williams Mountain. 

The second stage of activity may evidently be divided into two parts, and the succession of 
events is very similar to that determined for the second stage of eruption of San Francisco Moun- 
tain. During the first part the eruptions came from a single central vent. It is estimated that 
1 cubic mile of lava was thus erupted and that the cone was built up to a height of 3,200 feet. 
At this time the form of the volcano, to judge from the parts now exposed, was v^ry symmetrical, 
the slopes being slightly concave and smooth. The restorations show that the slopes of this 
cone are steeper than those of the andesite cone of the preceding stage, which expresses the more 
viscous condition of the dacite. During the later part of the second stage there was a dispersion of 
vents, and eruptions occurred from at least seven secondary orifices on the outer slopes or about 
the base of the main cone. They added nothing to the height of the main cone and only slightly 
increased its volume. In all, however, 1.2 cubic miles of lava was erupted from secondary vents, 
most of it from those at the base of the cone, making the total volume of dacite 2.2 cubic miles. 
From the forms of the cones it is inferred that the lava of the later eruptions was more viscous 
than that of the earlier outbreaks. In the absence of any change in the composition of the lava, 
this difference suggests that the temperature of the magma decreased as volcanic activity came 
to a close at this locality. The eruptions of the entire second stage in general were quiet, 
although at least one explosive phase is revealed by the pumice on the east side of the 
mountain. 

VOLUn OF THE COHX AVD OF THE LAVAS OF THB TWO STAGES. 

The erosion of the volcano has been so slight that the former outline may be easily restored 
and a basis secured for calculating the volume. For this purpose the three cross sections (fig. 12) 
have been used in the same manner as those of San Francisco Mountain. The volume of the 
cone built up by eruptions from the central main vents is as follows : 

Volume ofcoTie of Bill Williams Mountain. 
Cone generated by — Cubio miles. 

Section A-A' 1. 64 

Section B-B^ 2. 00 

Section C-C' 1. 87 

Average 1. 8 

The volume of the andesite cone is as follows : 

VoluTne of andesite cone of Bill Williams Mountain. 
Cone generated by — Cubic miles. 

Section A-A' 0. 63 

Section B-B' 100 

Average 8 



oYeaby peak. 63 

The volume of dacite erupted from the main vent is 1 cubic mile, the diflFerence between 
the above two averages. To this must be added the lava of the lateral vents on the outer slopes 
and about the base of the mountain, which is roughly calculated at 1.2 cubic miles, making a 
total volume of 2.2 cubic miles of dacite. 

PBOPORTIOir OF COHX BBODBD 8ZVCB CB8SATI0N OF VOLCANIC ACTIVITY. 

The same proportion of the cones generated by sections A-A' and B-B' has been eroded 
and equals 7 per cent of the total volume. This nearly represents the minimum amount of 
erosion. The maximum amount is shown by section C-C', and the proportion of the correspond- 
ing cone that has been removed is 14 per cent of its total volume. It is estimated from the 
topographic map that a fair average is obtained by giving the minimum value a weight of 2^ 
and the maximum a weight of 1. On this basis the proportion of the cone eroded since activity 
ceased is 9 per cent of the total volume. This is practically the same value as that obtained for 
San Francisco Mountain and Kendrick Peak. 

AGB OF THE COHX. 

The lavas of Bill Williams Mountain overlie on the west side the basalt of the first general 
period of eruption, and underlie on the north side the basalt of the third general period. They 
are therefore intermediate in age. As the cone has been eroded to the same proportionate ex- 
tent as San Francisco Mountain, and as the lavas at the two volcanoes are similar, Bill Williams 
Mountain should clearly be assigned to the second general period of eruption. 

8T7MXART. 

Bill Williams Mountain is a composite cone built up by lavas and breccias of two separate 
stages of activity. The eruptions came in part from central vents, in part from lateral orifices 
on the outer slopes or near the base of the cone. The eruptions from the lateral orifices deformed 
what would otherwise have been a cone as symmetrical as those of San Francisco Mountain and 
Kendrick Peak. At the close of activity the volcano was 3,200 feet high and covered an area 
of about 25 square miles. The volume of all the lavas was 3 cubic miles. 

Erosion has lowered the height of the cone on the average about 1,300 feet. The southern 
rim is eroded the most, and two large ravines on the southwest and northwest sides extend to 
the center of the cone. Alluvial fans mantle the lower slopes and are now undergoing dissec- 
tion. Of the total volume of the volcano 9 per cent has been eroded since it became extinct. 

The stages of eruption may be tabulated as foUows: 



stage. 


Type. 


Lava. 


Volume. 


Height 

of 0006. 


1 


1 

I 

PrMomlnftPtly explofilvft, ftctfv«. ai^d vteoix>tM ! AndftsitA . , _ . 


Cu. mila. 
0.80 
2.20 


Feft. 
2,100 


2 


PTfldomftiftxitlv o^iiet. ftotiv(». ftnn vteorous T)iw»ite . , , 


3,200 







The cone rests on the eroded basalt-covered surface of the cherty Kaibab limestone. 

The lavas were erupted after the basalt of the first general period of eruption but before 
that of the third period. They are assigned to the second general period of eruption because 
of their similarity to the andesites and dacites elsewhere in the region and because the cone of 
Bill WiUiams Mountain has been eroded to the same proportionate extent as the cones of San 
Francisco Mountain and Kendrick Peak. 

o'leary peak. 

topography. 

O'Leary Peak lies 9 miles northeast of San Francisco Mountain and is the easternmost of 
the lai^e cones. The highest peaks rise 8,925 feet above sea level, or about 2,500 feet above 
Deadman Wash on the northwest. On the south and east, however, surrounding basalt cones 
reduce the relative height of the mountain to about 1,800 feet. 



64 SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 

In views from the north and south the profile of the mountam is double (PL I, A, p. 16), 
owing to the overlapping of two separate cones. From the west, when the smaller cone is hidden 
behind the larger, the moimtain appears to be a single volcano. The outlines of the two parts 
are symmetrical and approximately conical in shape. The slopes of both cones, except at their 
extremities, average 25°. The slopes of the larger cone, however, are slightly concave, whereas 
those of the smaller one are slightly convex and the summit is dome shaped. The cones are 
bare of vegetation and are covered with a thin mantle of disintegrated lava through which 
ledges protrude at many points. The summit of the larger cone is marked by two peaks that 
are separated by a narrow saddle 200 feet lower in elevation. The two principal ravines of this 
cone are on the north and south sides and head at this saddle. 

It is estimated from restored outlines made on photographs of the larger and more recent 
cone that the height of the mountain has been reduced about 700 feet. This is considerably 
less than the reduction of 1,300 feet determined for Bill Williams Mountain, which is of about 
the same size. The impression derived from field views is that O'Leary Peak has been less 
eroded. The rainfall is less at O'Leary Peak than at Bill Williams Mountain because the 
locality is farther from the edge of the plateau and is also cut oflF from the rain-bearing winds 
by San Francisco Mountain. It is probable that this climatic condition has existed for along 
period and that it may account for the apparently less eroded state of this cone. 

ERirPTIVE HISTORY. 
YARIETISS OF LAVA. 

O'Leary Peak is formed by two cones composed of different lavas and representing distinct 
stages of activity. So far as known only lava is present at this locality, and its distribution 
shows that most of it was erupted from central vents. There are, however, some small masses 
at the southwest base of the mountain that came from secondary orifices. The lavas in order 
of eruption are rhyolite and dacite, having the composition shown by the following partial 
analyses: 

Partial analyses of lavas fnmt 0*Leary Peak. 



Rhyolite. 



Dacite. 



BiOt.. 

AlfOt. 

FeiO». 

FeO.. 

MgO. 

CaO.. 

NatO. 

KsO.. 

TiOs.. 



67.0 


62.3 


16.5 


16.4 


2.3 


2.9 


2.1 


3.3 


.5 


2.1 


2.0 


3.8 


5.1 


4.3 


8.3 


8.2 


.4 


1.0 



99.2 



99. 3 



Both lavas are megascopically counterparts of the rhyolites and dacites that occur else- 
where in the region. They are also mineralogically similar to these rhyolites and dacites. 
Chemically, however, they are somewhat more basic, especially the rhyolite, on account of 
regional differentiation. The relationship of these lavas to similar but more acidic lavas is 
evident and places them in the second general period of eruption. 



FIRST STAGE OP ERUFHON. 



The rhyolite of the first stage of eruption is found in two cones of unequal size. The larger 
cone forms the eastern part of the main mass of O'Leary Peak; the smaller one is situated on 
the edge of the O'Leary lava field 2 miles S. 75° W. of the summit of the mountain. The 
rhyolite also occurs in flows which cover about 2 square miles north of the mountain. Flows 
may have extended in other directions, but if so they have been buried under the basalts of 
the succeeding period of eruption. The larger cone was not closely examined, but its dome- 
shaped form shows that the lava was extruded in a rather viscous condition. It is estimated that 
the original height of this cone was 2,700 feet and its volume 0.5 cubic mile. Only the summit 
of the smaller cone now protrudes above the surrounding dacite and basalt. The original height 
of this cone was about 1,200 feet and the volume 0.1 cubic mile. 



O^LEABY PEAK. 65 

The lava of the main rhyolite cone, where it was examined at the north foot of the mountain, 
is a finely banded dark-brown felsite. The rhyolite of the smaller cone, so far as exposed, 
consists of a black semilustrous glass and a light-gray aphanite, with intermediate varieties due to 
mixtures of the two. The obsidian was erupted after the lithoidal variety. 

SECOND STAGE OF ERUPTION. 

Dacite forms the western and greater part of the main mountain. The relation of the dacite 
and rhyolite cones was not determined. It is supposed that a part of the latter is buried beneath 
the former, as shown on cross section C-C' of San Francisco Mountain (PI. VI, p. 40). The 
height of the cone is estimated to have been 3,500 feet. The lava occurs in flows west and 
northwest of the mountain, where it forms a prominent mass with steep slopes rising 100 to 
200 feet above the level of Deadman Wash. The rock exposed in the saddle between the two 
highest summits and in the upper part of the southern ravine probably belongs to the core of 
the volcano. It weathers into small pinnacles, and in thin section the groundmass, although 
very fine grained, shows no sign of flow structure. The dacite at the southwest base of the 
mountain likewise exhibits no flow structure in thin section and is supposed to represent a 
small viscous eruption from a secondary vent. 

The lava shows the common tendency of the dacites of the r^ion to occur in heavy compact 
flows free from scoriaceous surfaces. It appears, in general, to contain a larger proportion of 
plagioclase phenocrysts; in some specimens they form one-third of the rock. Inclusions of & 
dark-colored microcrystalline igneous rock are very common in the flows on the west side of 
the mountain. In places they make up from one-tenth to one-half the area of small exposures. 
They were evidently taken up in a solid condition by the dacite, for the contacts are always 
perfectly sharp and well defined. 

V0LX7KX OF THB COHX AVD OF THB TWO LAVAS. 

A reliable calculation of the volume of the lavas of O'Leary Peak is not possible. Approx- 
imate results may be obtained, however, by considering the volcanoes as perfect cones with 25** 
slopes and heights as given in the preceding description. On this basis the volume of the two 
rhyolite cones is 0.6 cubic mile. In addition there is 0.3 cubic mile in flows, making a total of 
0.9 cubic mile for the lava of the first stage of activity. The volume of the complete dacite 
cone is 1.3 cubic miles, which is reduced to 1 cubic mile to allow for the portion of the rhyolite 
cone supposed to underlie it. The volume in flows is estimated at 0.2 cubic mile, making the 
total 1.2 cubic miles for the lavas of the second stage of eruption. The total volume of the 
lavas of O'Leary Peak, therefore, is 2.1 cubic miles. 



O'Leary Peak is a double cone built up by lavas during two separate stages of activity. 
The eruptions came predominantly from a single vent in each stage, but there were several 
minor orifices near the bases of the main cones. At the close of activity the volcano was 3,500 
feet high and covered an area of about 15 square miles. The volume of the two lavas was approx- 
imately 2.1 cubic miies. 

The dacite cone appears to have been reduced in height 700 feet. The slopes are covered 
with a thin mantle of disintegrated lava and there are no prominent ravines, such as occur on 
the other large cones. 

The stages of eruption may be tabulated as follows: 

Stages q/ eruption o/O^Leary Peak. 



Stage. 



1. 
2. 



Type. 



Qafet. 
..do.. 



Lara. 



Volame. 



Rhyolite. 
Dacite... 



Cu.mlUt. 
0.9 
1.2 



Height 

OfOOHB. 



Feet, 
2,700 
3,50D 



75008'*— No. 7&— 13- 



66 



SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 



The cones of both stages were very active and the eruptions vigorous. The rhyolite is 
supposed to have been decidedly viscous, as the cone has steep convex slopes. The dacite was 
less viscous than the rhyolite. 

The platform on which the cones rest is not exposed in the immediate vicinity. It may 
be the eroded basalt-covered surface either of the cherty Kaibab limestone or the Shinarump 
group. 

The lavas of O'Leary Peak are fixed by local evidence only as older than the basalt of the 
third general period of eruption. Their character places them in the second general period of 
eruption with those of the large cones already considered. 

SITOREAVES PEAK. 
TOPOGRAPHT. 

Sitgreaves Peak is situated 19 miles west of San Francisco Mountain in line with Eendrick 
Peak and Bill Williams Mountain. It rises about 9,300 feet above sea level, or about 1 JOO 

feet above the general level of Government 
Prairie on the southeast side. The crest of 
the mountain is irregular and varies in outline 
from different points of view ; the slopes, on the 
contrary, are very symmetrical and gently con- 
cave (PI. IX, C,p. 54). The inclination is 20** 
on the upper slopes and gradually decreases 
to less than 5^ at the base of the mountain. 
The maximum slope at the original summit 
appears to have been 25^, so that the moun- 
tain as a whole had somewhat flatter slopes 
than the other large cones. It is estimated 
that the height of the cone, since it became 
^ ,o „ «. ,ai. T, 1. ^ i .u *ot extinct, has been reduced at least 1,000 feet and 

FIOUBK 13.~Proflle8 0f Sitgreaves Peak: il, seen Irom the west; B, from ' . . ' 

the northeast; C, from the southeast; r, rhyoUte cones; b, basalt cones that erOsion haS been mOSt vigOrOUS On the 
2 and 5, corresponding peaks; +. middle point of base. ^^^^j^ ^j ^^^^ g.j^g rpj^^ ^^j^j material 

now mantles the lower slopes in large alluvial fans, which are trenched to their outer edges 
by numerous washes, in the same manner as the alluvium of Bill Williams Mountain. 

8TRVCTT7BB OF THE MADT CONS. 

The main mass of Sitgreaves Peak was not studied in sufficient detail to permit any exact 
conclusion as to its structure. Restorations made on photographs show that the mountain is 
most probably a simple cone built up about a central vent (fig. 13). The profiles were completed 
by first restoring the upper slopes, the summit or crater being given a diameter of 1 ,000 feet. The 
lower slopes were then projected downward to the inferred level of the platform on which the 
cone rests. The middle point of the base in each section, marked by a cross, falls within the 
assumed position of the crater, and this coincidence suggests a simple structure for the cone. 
A detailed field examination might disclose minor lateral vents on the slopes such as were 
associated with the main orifices at the other large cones. An outcrop of agglomerate con- 
taining fragments of sedimentary rocks, which is exposed at the southeast base of the cone 
near point 6 (fig. 14), probably represents an initial outbreak from a secondary vent. It is 
situated about 600 feet above the inferred level of the platform on which the main mass of the 
cone rests, and at this altitude elsewhere on the slopes only lava is found. 

EBtrPTIVE HISTO&T. 

Sitgreaves Peak consists wholly of rhyolite and in this respect differs from all the other 
large cones. The eruptions built up one large cone to a height of 3,500 feet and three smaller 
ones from 1,000 to 1,500 feet high. Two of the latter are situated at the base of the main cone, 




SITGBEAVES PEAK. 



67 



as shown in figure 14 (A and B) ; the third is located 5} miles N. 85^ E. of the summit of the 
mountain. 

The lavas of the main cone consist for the most part of light-brown and bluish felsites 
and a subordinate amount of black obsidian. In appearance they are the counterpart of the 
rhyolites that occur elsewhere in this region and are clearly to be correlated with them in the 
second general period of eruption. The chexnical composition of the lava may be seen from 
the f oUo wing analysis : 

Analysis of rhyoliU from Sitffreaves Peak. 



AI3O, 

FeO. 
MgO. 
CaO.. 



74.9 
13.1 
.5 
.8 
.2 
.3 



NajO. 
TiO, 



6.6 

4.3 

.3 

.1 



100.1 



SitgreavesPk \ "^ ^ ./ . . 
\ • Cone ; 



■^-. 



*^...» 



The lavas of the small cone marked A in figure 14 dip away on aU sides from a central 
point, where there is a mass of agglomerate composed of fragments of lavender-colored lava 
embedded in a lighter-colored cement. Evidently this is the core rock, and its distribution 
shows that the crater was not more than 300 feet in diameter. The cone, which now rises 500 
feet above the level of Spring Valley, has lost some of its original height, though it is still very 
symmetrical in form. The slopes are almost entirely 
covered with loose fragments of lava and are cut by 
many ravines which extend with low grades nearly to 
the center of the cone. The lavas consist of three varie- 
ties — a dense pinkish felsite, a light lavender-colored 
spherulite, and a lustrous black obsidian. In general 
the felsite predominates in the earlier flows, the 
spherulitic and glassy varieties in the later ones. 

Cone B (fig. 14) is composed entirely of the felsitic 

type of rhyolite, rather porous in texture and ranging 

in color from very light gray and pink to dark bluish 

gray. The rim of the cone is broken down on the oast 

side, and the principal flows appear to have occurred in 

that direction. The third small cone contains varieties 

of rhyolite of the same character as those in cone B and 

in addition a glass finely banded in black and very dark-gray layers. There are also between 

cone B and the third cone several isolated knobs of rhyolite, surrounded by basalt, whose point 

of origin was not determined. They are too small to be shown on the general geologic map 

(PI. Ill, p. 20). 

voLun OF THB con. 

 

The volume of the cone can not be definitely calculated. On the assumption that the cone 
had an original height of 3,500 feet and slopes as indicated by the restored outlines made on 
photographs, it is estimated that the volume was between 2 and 4 cubic miles. An average 
value of 3 cubic miles may be taken, which includes the volumes of the small cones. 

PBOPOBTION OF THB CONE EBODBO 8ZVCE CBSftATIOir OF VOLCANIC ACTIVrPT. 

Tentative calculations of the proportion of the cone that has been eroded have been made 
on the assumption that all po'uits in the cross sections drawn on photographs are in the same 
vertical plane. On this basis 10 per cent of the total volume of the cone generated by section 
A has been eroded ; 5 per cent of the cone corresponding to section B ; and 2 per cent of the cone 
of section C. If the maximum value is given a weight of 2 and the diiddle value a weight of 1, 
an average is obtained of 7 per cent of the total volume of the cone. Although no especial 
accuracy is attached to this method, the results obtained are of the same order of magnitude as 



FiQUBE 14.— Di&graxnmatlc plan of Sitgreaves Peak and 
adjacent cones. 1 to 5, Points on main cone; A, B, 
smaller rhyolite cones. 



68 



SAN FRANCISCAN VOLCANIC FIELD, ARIZONA. 



\ 



N. 



those for San Francisco Mountain, Kendrick Peak, and Bill WUliams Mountain, which possess 
a fair degree of precision. The agreement is close enough to confirm the field impression that 
Sitgreaves Peak has been eroded to the same extent as the other large volcanoes and that it 
undoubtedly belongs in the second general period of eruption. 



MORMON MOUNTAIN. 



TOPOGRAPHT. 



Mormon Mountain lies on the Black Mesa 28 miles south of San Francisco Mountain. It 
occupies an isolated position with respect to the other large cones, although its distance from 
San Francisco Mountain is less than that of Bill Williams Mountain. Its general characters 
show that it should be included in the second period of eruption with the large cones to the 
north. It is the smallest of the composite cones, rising 8,600 feet above sea level, or not more 
than 1 ,500 feet above the surrounding country: The mountain is irregular in form and elongated 
in nearly a north-south direction. It is designated Mount Longfellow on the United States 
General Land Office map of Arizona, but it is commonly known as Mormon Mountain, because 
a Mormon settlement was formerly located on the shore of Mormon Lake at its eastern base. 
Only the south slope was visited and a hurried ascent made to the summit. 

BBXTPTIVS HISTORY. 

Two distinct stages of activity are indicated by the presence of a latite and a dacite of the 
following (partial) chemical composition: 

Partial analyses of lavas from Mormon Mountain. 



2. Dacite. 



SiOt.. 

AI2O3. 

FeiOa 

F«0. 

MgO. 

CaO. 

NasO. 

KsO. 

TiOj. 




The megascopic characters of the two lavas point to their identity with the corresponding 
lavas of the other large volcanoes. The latite is duplicated in appearance* by some of the flows 
of the first stage of eruption at San Francisco Mountain and especially by the lava of Observatory 
Mesa, near Flagstaff. The dacite is the counterpart of the equivalent lavas found elsewhere in 
the region. 

By far the greater part of the mountain appears to be composed of coalescing cones of 
latite in thick and rather short flows. The lava is light to dark gray in color and commonly 
has a platy structure. Its aphanitic groundmass contains a few small phenocrysts of hornblende. 

The dacite was observed as an eroded knob on the southwest side of the mountain at an 
elevation of 8,200 feet. Its relation to the andesite was not definitely determined. The lava 
dips southward coincident with the slope and may be considered either an eroded portion of a 
cone partly buried by later flows of latite or a small flow that broke out on the slope of the latite 
cone. The dacite is somewhat fresher in appearance than the latite and on this account is 
believed to represent a later stage of eruption. 

VOLTTXB OF THB CONE. 

No definite figure can be given for the volume of the lavas. It has been roughly placed 
At 2 cubic miles, of which 1.5 cubic miles is latite and 0.5 cubic mile dacite. 



^v 



SAK FBANCISCAK VOLCANIC FIELD, ABIZONA. 69 

OBSERVATORY MESA. 

Observatory Mesa extends from Fort Valley to the vicinity of Flagstaff, where the Lowell 
Astronomical Observatory is situated on its south end. It is 5 miles long and between 2 and 3 
miles wide. The surface of the mesa has an elevation of 150 feet above Fort Valley and slope^' 
to the southeast at an angle less than that of the surrounding country, attaining at its south 
end a relative altitude of over 300 feet. An irregular cliff of no great height bounds the mesa 
on all sides, below which is a generally well-developed talus slope. The mesa is formed entirely 
of lava erupted from Crater Hill, a cone 500 feet high located near its north end, and from a 
much smaller cone just to the east. The upper surface of the flows is somewhat decomposed 
and covered with a thin mantle of soil, through which protrude small bare knobs of rock. 

The lava composing the cones and mesa is uniformly dense, commonly showing a platy 
structure, and is of a dark-gray color. In hand specimens it is rather basaltic in appearance. 
The specific gravity of specimens from different parts of the mass is as follows: 

Specific gravity of lava from Observatory Mesa. 

Crater HUl 2.71 

Edge of mesa, north end 2. 60 

Edge of mesa, near Anderson's ranch 2. 66 

Edge of mesa, near observatory 2. 68 

Edge of mesa, south end 2. 64 

Average 2.65 

These determinations have a variation of only 4 per cent and indicate the homogeneity 
of the mass. The densest lava is located at the point of eruption. 

The andesite of San Francisco Mountain is in close contact with the lava of the mesa along 
its northeast side and has bleached it to a notable extent. This fact, taken in connection with 
the specific gravity and other characters of the lava, makes it certain that the lava of Observatory 
Mesa is equivalent to the latite of the first stage of eruption of San Francisco Mountain. 

SUOABLOAF HILL. 

Sugarloaf Hill is a small rhyoUte cone situated on the east slope of San Francisco Moimtain. 
The summit rises 9,281 feet above sea level, or on the average 1,000 feet above its base. A 
second mass of closely related rhyolite adjoins Sugarloaf Hill on the north. The two masses 
show very plainly in views of San Francisco Moimtain from the east (PI. VII, A) on account 
of their light color. 

The summit of Sugarloaf Hill contains a roughly circular depression about 300 feet in 
diameter, representing the remains of a crater. The lava which now forms the rim rises less than 
100 feet above the bottom of the crater, but it must originaUy have risen to a greater height, 
as the cone shows signs of considerable erosion. The flow structure of the rhyolite in the rim 
is very clearly marked and almost without exception dips toward the center of the depression 
at angles of 50° to 90°, the average of six observations being 65°. The rhyolite of Sugarloaf Hill 
has throughout a uniform texture, well described as pasty, and is very light gray or brown in 
color. Alternations of these two colors produce the banded appearance of the rock. The groimd 
mass is aphanitic and contains small phenocrysts of quartz, feldspar, and biotite. 

The rhyolite north of Sugarloaf HiD, which was erupted from an independent vent, is a 
low, rather dome-shaped mass. The early flows have a uniform stony texture and are Ught gray 
in color; the later ones have a pasty texture and are brown in color. An intermediate variety, 
formed by the alternations of the two main types, shows that the change from the first to the 
second took place gradually. 

The rhyoUte at this locality was erupted before the andesite of San Francisco Mountain, 
as that lava overUes it on the south side of Sugarloaf Hill. It also appears to antedate the dacite 
of the second stage of activity, which abuts against the smaUer mass of rhyolite and bulges out 






70 BAN FBAKCISCAN VOLCANIC FIELD, ABIZOKA. 

to the east around its north end in a manner strongly suggesting that it met an obstruction to 
the normal northeastward course of flow (PL V, p. 40). If this interpretation is correct, the 
rhyoUte of Sugarloaf Hill was not contemporaneous with the rhyoliteof the fourth stage of activity 
of San Francisco Moimtain, although the two vents are but 4 miles apart. 

DBT LAKE HILLS. 

The Dry Lake Hills lie between San Francisco and Elden mountains and consist of about 
five small cones of dacite. The lava is similar to the dacites foimd elsewhere in the region, but 
was probably more viscous, as the cones are distinctly dome shaped. Two of the larger cones 
have crater-like depressions at their summits, and a shallow lake occupies one of these depres- 
sions when conditions are favorable. 

The dacite overlies the basalt of the first general period of eruption at a point half a mile 

northeast of the Flagstaff reservoir. It also appears to have been erupted after the formation 

of Elden Moimtain. This relation may be seen at the northwest foot of Elden Moimtain, a 

^ g little over half a mile north of Doyle's spring, 

I s where the two lavas approach the closest to 

i 5 one another (fig. 15). A narrow tongue of 

dacite from the Dry Lake Hills dies out 75 

,Q. .= i 5 ^.^59i*T'+*v*J* foet from the base of Elden Moimtain. It 

/>^^t>^j7?^v^ " • ~ ^<i^+I+^*^^+ ' appears less eroded than the lava of Elden 

Mountain and is therefore considered younger. 

The irregular diagonal boimdary of the dacite 

of the Dry Lake Hills on the northward- 

59 Feet sloping surf acc of the northern sedimentary 

FtOTTBX 15.— Cross section of valley half a mUe north of Doyle's spring, block of Elden Mountain IS interpreted aS 
showingrelationoflavasof Dry Lake HUls and Elden Mountain. ••£_■ xi. ix- * xic 

sigmfymg the same relation. A mantle of 
alluvium effectually hides all other contacts and makes it impossible to say whether the lavas 
of the Dry Lake Hills were strictly contemporaneous with the dacites of San Francisco 
Mountain; but in view of their exact similarity it may be assumed that they were erupted 
during the second or third stage of activity of the San Francisco volcano. 

LACCOLITHS OF THE SECOND PERIOD. 

» 

Three masses of igneous rock are included in the laccoUthic group, but only one of them, 
Marble Hill, is a true laccolith. Elden Moimtain, which approaches the large cones in size, 
exhibits in a unique manner laccolithic intrusion and volcanic extrusion as contemporaneous 
phenomena in the same geologic unit. The origin of Slate Mountain is doubtful because of 
insufficient field work; it may be similar to that of Elden Mountain. Though these masses 
possess no little interest in themselves, they have a specific local interest on accoimt of the 
presence on the flanks of Elden Mountain and Marble Hill of upturned lavas from San Francisco 
Mountain, by means of which their position in the sequence of events in the San Franciscan 
volcanic field may be definitely determined. 

MARBLE HILL. 

Marble Hill is ia, normal laccoUth of small size situated on the lower northwest slope of San 
Francisco Mountain, 3^ miles from San Francisco Peak. It rises 9,065 feet above sea level, 
but as the surrounding coimtry has an elevation of 8,000 to 8,500 feet, the relative relief is 
only 800 feet. The base, as marked by the enveloping mantle of waste, is nearly circular in 
form and covers an area of 1.5 square miles. The hill consists of a central peak, which is the 
main simimit, almost completely encircled by a series of hills and ridges whose elevations range 
from 8,500 to 9,000 feet. This circle of hills Ues from a quarter to half a mile from the central 



MABBLE HILL. 



71 



peak and is separated from it by a marked depression. Its continuity is imbroken except at 
two points on the north side, where the interior drainage escapes into Deadman Wash (fig. 16). 
The simunit of the central hill is composed of a granite porphyry, delicate gray in color and 
with a uniform compact texture. An aphanitic groundmass contains a few small phenocrysts of 
feldspar and biotite. The intrusive nature of the porphyry is very evident, as the limestone 
with which it is in contact has been completely changed to a coarse-grained white marble. 
A series of sedimentary and effusive igneous rocks, which forms the remainder of the hill, overlies 
the porphyry. A limestone outcrops on the lower slopes of the central peak and on the north- 




Top«<raphy by /\ 
H.H.Robin«et\1903U 



1000 



2000 



3000 



4000 



5000 feet 



Contour interval 100 feet 















X 




A 




•34. 



Gnmite porphyry 
(tntnunw) 



FROlt 



1%row of fkult 



FVactnres 



Strike and dip 

sedimentary strata 

and volcanie lava flows 



Located 

observatioQ 

point 



Figure Id.— Topographic and preliminary structure map of Marble Hill. A- A', Line of section in fig. 17. 

west side rises nearly to the same height as the igneous rock. Above the limestone, strati- 
graphically, and occupying the lower groimd between the central peak and surroimding hills, 
are sandstones. The lower ones are red in color,. rj9,ther soft, and even-bedded; the upper 
ones are white and cross-bedded. A cherty limestone rests upon the cross-bedded sandstone, 
and overlying this limestone are lava flows, w^hich cap the summits of all the higher encircling 
hills. The sedimentary series is made up of Carboniferous formations. The lavas belong to 
both the first and the second general periods of eruption. The measurement of the section is 
made somewhat difficult by the faulting and crushing of the strata. The foUowing table gives 
the formations present, in descending order, and their average thickness. 



72 SAN FRANCISCAN VOLCANIC FIELD, ABIZONA. 

Section nuatured at Marble Hill. 

Pyroxene dacite 150 

Latite 10 

Basalt (lava and ash) 130 

Kaibab limestone (cherty ) 240 

Coconino sandstone (cross-bedded) 670 

Supai formation (red sandstone) 670 

Redwall limestone 180 

1,960 

The sedijnentary strata, with the overlying lavas, have been upturned to very high angles 
by the intrusive rock, as may be seen from the recorded observations on the map (fig. 16). The 
average dip is 60°, and three-quarters of the measured outcrops have dips of 60° or greater. 
The strata are concentrically arranged about the igneous core, as is plainly brought out by the 
strike observations recorded on the map. The position of the core, however, is not exactly at 
the center of the circle of outlying hills, but is rather nearer the southeast side. This indicates 
the slightly unsymmetrical character of the intrusion, which has tilted the strata more steeply 
on the south than on the north side of the core. 



NW 







5a *£^^^ 

XSW^:- 

***"*»««^ *•■■■■■ ■%-- ••••^.....^ 

! ~ - :2z.^.^ 

I • 

t 


•r.rt:.':--— " 6. ' .^ • / 




 — • '. ..-^y 

-:;:r:.« •-^■■''''^ 


.._.~....»..*— — — — — .......———*" 


• 



1000 tOOO 3000 4000 SOOO PMt 

I I I I i I 



FlQUBX 17.— Geologic cro« section through Marble Hill: 1. AUuvlum; 2, basalt of third period; 3, pyroxene dacite; 4, latite; 5, basalt of flnt 
period; 6, Kaibab limestone; 7, Coconino sandstone; 8, Supai formation (red sandstone); 9, Redwall limestone; 10, granite porphyry. For 
Une of section see figure 16. 

Radial faulting and fracturing are most easily recognized and the position and direction 
of throw of fourteen such planes of dislocation are indicated on the map. They all extend at 
least to the foot slopes of the hill and most of them radiate directly from the igneous core. 
Although in a strict sense the cover is divided into fourteen blocks, if fractures are disregarded 
and successive blocks with the saxne direction of throw be grouped together, a system of eight 
blocks remains in which alternate blocks are either raised or lowered with respect to each other. 
The faults that bound these blocks are marked on the map by crosses at their outer ends. Con- 
centric faults are also present, as shown by the contiguity of strata belonging to different hori- 
zons. It seems reasonable to suppose that there should be a considerable number of such 
faults. They are difficult to locate, because they are parallel to the strike of the strata and 
their throw may not be sufficient to bring diverse beds into contact. The dislocations in the 
cover of the Marble Hill laccolith have been duplicated experimentally by Howe,* who says, in 
a discussion of variations dependent on passive agents in laccolithic intrusion: 

Fractures initially radial and secondly concentric tend to form in frangible strata over a symmetrical dome, the 
ladial fracture* gaping upward and the ci>ncentric ones downward. 

The structure of the laccolith along a northwest-southeast line through the summit is shown in 
cross section A- A' (fig. 17\ and may be briefly described. On the southeast side of the igneous 

I Hov«, Eraest, £xperim«)t» illustrating intrusioD and eroeian: Tvcnty-fi^^t .Knn. Rept. U. S. Geol. Survey, pt. 3, 1901. p. 302. 



MABBLE HILL. 73 

core the Redwall limestone dips 80° SE., the basalt which caps the encircling hill dips 70°, 
and the dacite at the outer foot slope has a dip of 50°; this shows the extent to which the 
doming dies out on this side in a distance of 1 ^ miles. The dip on the northwest side is 45° in 
the Redwall limestone, though in the overlying formations it is from 35° to 50°. Along this 
particular section there is thus a marked difference in the dip of the strata on either side of the 
intrusive core. The apparent thickness of the red sandstone of the Supai formation, northwest 
of the porphyiy, is but 200 feet, whereas the actual thickness is placed at 670 feet. The dis- 
crepancy is due to faulting, which has raised the beds on the side next to the porphyry. The 
fault plane is shown in figure 17 with a westward dip to express the idea that the beds were 
sheared by the unsymmetrical intrusion of the igneous rock. The strata on the southeast side 
of the core, from being upturned to nearly vertical angles, have suffered a great contraction in 
thickness. Formations that have a thickness of 1,950 feet on the northwest side of the core 
are but 1,100 feet thick on the southeast side, the difference being 40 per cent of the normal 
thickness of the section. As the force of intrusion was exerted largely in a vertical direction, 
the contraction must have been produced for the most part by shearing and faulting closely 
parallel to the bedding planes. 

Any attempt to restore the eroded portion of the cover will show that it must have been 
greatly shattered and that a favorable opportunity was presented for the escape of the igneous 
material in the form of surface flows. The fact that it did not so escape indicates a nice balance 
between opposing forces and suggests that the magma was distinctly viscous. The high vis- 
cosity of the igneous mass is most plainly indicated, however, by the absence of dikes, although 
the numerous radial and concentric fault planes presented favorable conditions for their intru- 
sion. The shape of the igneous mass is clearly pluglike in its upper part, and the mass might 
be considered as representing a transition form between a normal domed laccolith and a vol- 
canic neck. What would have happened to the cover if the igneous body had established free 
connection with the surface is of course purely conjectural. Certain possibilities, however, 
suggest a structure not unlike that postulated by Von Buch in his theory concerning *' craters 
of elevation.'' 

The eroded portion of the cover is not restored on the cross section (fig. 17), because no 
satisfactory idea of its original condition can be formed. The representation of the lower part 
of the igneous mass is of course purely conventional and designed simply to show its laccolithic 
nature. It has been suggested that the igneous rock is throughout pluglike in form. This, 
however, is impossible, because the Redwall limestone at the points where it outcrops at the 
surface of the country is 4,000 feet above its normal position. As a topographic map was not 
available when the locality was visited, the work was not so detailed as was later shown to be 
desirable. The Marble Hill laccolith, on account of its small size and the completeness with 
which the phenomena of intrusion are exhibited, offers an interesting field for further study. 

Marble Hill may be very closely correlated with San Francisco Mountain, and its position 
in the sequence of events in the volcanic field thus fixed, by means of the upturned lavas upon 
its flanks. These in ascending order are basalt, latite, and pjrroxene dacite. The basalt, 
which is the same as that found on the north flank of Elden Mountain and at many other localities 
in the region, belongs to the first general period of eruption of the San Franciscan volcanic field. 
The latite and dacite are the same in all respects as the corresponding lavas in San Francisco 
Mountain, and imdoubtedly originated at that volcano during the first and second stages of 
activity. In addition to the above lavas, two dikes are exposed on the west and south sides 
of the central peak. One is composed of typical pyroxene dacite. The rock of the second 
dike, south of the summit, is essentially the same as that of the first, but contains phenocrysts 
of hornblende and biotite instead of pyroxene. This feature, it is believed, marks it as equiva- 
lent to the hornblende dacite of the third stage of activity in San Francisco Mountain. As the 
granite porphyry of Marble Hill is almost identical in chemical composition with the rhyolite 
of San Francisco Mountain, it seems reasonable to regard these rocks as contemporaneous and 
to suppose that the laccolith was formed during the fourth stage of eruption in San Francisco 



74 SAN FRANCISCAN VOLCANIC FIELD, ABIZONA. 

Mountain. The absence of the andesite of the fifth stage of activity on the slopes of Marble 
Hill tends to confirm the correctness of the above correlation, for had the intrusion occurred 
after that stage the andesite would very probably be found upturned with the older lavas. 

The evidence indicates very clearly, then, that the Marble Hill laccolith was completely 
formed during a single stage of activity in San Francisco Mountain. This stage — the fourth — 
was perhaps the shortest in the history of the volcano, so far as the time necessary for the 
eruption of the lava was concerned. The entire length of the period from the close of the third 
to the beginning of the fifth stage is unknown, but it must have been very brief in a geologic 
sense. 

ELDEN MOUNTAIN. 
SXTXTATIOn AVD TOPOOaAPHT. 

Elden Mountain is situated at the southeast foot of San Francisco Moimtain 4 miles 
northeast of Flagstaff. It rises in its highest summit 9,280 feet above the sea, or 2,400 feet 
above the surrounding country on the south and east, and covers an area of about 10 square miles. 
The greater part of the mountain consists of igneous rock in dome-shaped masses. There are, 
however, extensive areas of sedimentary strata on the east and north sides that are intimately 
associated with the igneous rock and constitute an integral part of the mountain. It may be 
said that the mountain displays in an unusual manner laccolithic intrusion and volcanic extrusion 
as contemporaneous phenomena in the same geologic unit. 

The extent of the erosion of the mountain varies at different localities, depending on the 
character and position of the rocks. It has been greatest in the eastern area of sedimentary 
strata; these are now surrounded on all sides but the east by a higher rim of igneous rock, 
although formerly the reverse relation held true. Erosion of the igneous rock has been extensive 
in the space between the two sedimentary areas, where a breccia is present, but outside of this 
locality it appears to have been relatively slight. The extent of erosion on the southwest 
slope of the moimtain and the manner in which it is proceeding may be seen in Plate X, A. 
The slopes are scored by sharply incised ravines of youthful appearance, which by headward 
and lateral cutting are lowering the height of the mountain and consuming the slopes. Alluvial 
fans spread out from the mouth of every ravine; they head from a third to half way up the slope, 
depending on the size of the ravine, and, coalescing at the base of the mountain, entirely sur- 
round it. These features are common to all the large cones of the second general period of 
eruption, and the field impression is that Elden Mountain has been eroded to proportionally 
the same extent as the large cones. 

lOXTEOirS HOCK. 

The igneous rock is throughout a dacite, identical in mineral and chemical composition 
with the dacites of San Francisco Mountain. It is mineralogically the same as the lavas of 
Schulz Peak and the Dry Lake Hills and is presumably chemically the same as those rocks, 
although they have not been analyzed. 

The chemical composition of the rock is given by the following analysis: 

Analysis of dacite from Elden Mountain. 

SiOj 65.9 iNa^O 4.5 

AI2O3 , 17.1 I K2O 3.1 

FejOa 4.7 HaO 4 

FeO : 2 IxiOj 5 

MgO 9 I 

CaO 2.6 . 99.9 

The small amount of ferrous iron indicates the complete alteration of the dark minerals, 
a condition characteristic of the mass. The mineral composition is constant, the rock con- 
sisting of 8.5 per cent of dark constituents and 19 per cent of plagioclase feldspar (Ab^Ani) 
as phenocrysts in a predominantly feldspathic holomicrocrystalline groundmass. The 



ELDEN MOUNTAIN. 




SCOMENTAAV.ROCKS 



CARBONIFEROUS 



Permian? PeTtraj^ranKBi 



Sxcmd period <if ervptum 



Fintptriod 
tjf eruption 



tnii 



(Thebourutartea torniaV& 



Kalbab Undivided DactlelnElden Daciteln LuTaandbracctai BlwalL 
limes lone fomiBiionB Mountain. SanFVanclBca InSnjiFmndioa. 

tefe " ' ' 



s MOtloDi In flfuna lV-31, 3S. 



.—Geologic map ol Eldeo UcHlIitain. 



76 SAN FBANCISCAN VOLCANIC FIELD, ABIZONA. 

groundmass exhibits almost no evidence of flow structure. This is in distinct contrast to the 
conspicuous flow structure shown by the hypercrystalline groundmasses of the dacites at all 
the other localities in the region with the partial exception of O'Leary Peak. The rock has 
a uniform texture and in large masses shows an apparent megascopic banding which is best 
brought out by differential weathering (PL X, B). Scoriaceous surfaces are entirely lacking 
and the rock may be said to possess a massive appearance. It was presumably this feature 
that led Lieut. Whipple ^ to speak of Elden Mountain as a '*huge mass of red granite." 

The most evident feature of the igneous rock, however, is the jointing, which is developed 
on a much larger and more pronounced scale than in the dacites. elsewhere. The intersecting 
joint planes form irregular columns, having a maximum diameter of 20 feet, with roimded tops 
produced by greater weathering along the joints (PI. X, B). The joints at any one locality 
vary in direction, but generally two principal planes may be distinguished which intersect at 
about 90*^ and have vertical or steep dips. At some places there is also a third plane parallel 
to the slope, and the intersection of this plane with the first set produces obliquely truncated 
columns. Examples may be seen on the eroded southern edge of the slope east of the northern 
summit and also on the south side of the mountain. Whether this third plane is of primary or 
secondary origin was -not determined, nor was the study of the joint system broad enough to 
permit a statement of its relation to the igneous mass as a whole. Between the northern summit 
of the mountain and the edge of the sedimentary block to the south the igneous rock is not so 
thoroughly jointed; it appears more massive than elsewhere and has been eroded into prominent 
pinnacles. 

The dacite of Elden Mountain has the same chemical and mineral composition as the other 
dacites of the region. Its distinctive features have not resulted, therefore, from any pecuHarity 
in these respects, so that they must be due to special conditions in regard to the temperature 
and rate of cooling of the mass. 

SSDDCZHTABT HOCKB. 

The sedimentary rocks occur at two distinct localities, one on the east and the other on the 
north side of the mountain. They will be described separately. 

These are the only locaUties at Elden Mountain where the strata are found upturned by 
the intrusion of igneous rock. Elsewhere outcrops of sedimentary rocks are very scarce about 
the immediate foot of the mountain. On the south side horizontally bedded Kaibab Umestone 
approaches within a mile, and on the west the basalt-covered Moencopie formation, also hori- 
zontally bedded, is distant about half a mile from the base of the mountain. 

EASTERN SEDDfENTARY AREA. 

The exposures are more extensive and represent a larger number of formations on the east 
side of the mountain, where they cover an area of about 1 square mile. The formations present 
(see pp. 20-37) in descending order, are as follows: 

Feet. 
Kaibab limestone (top eroded) 300 

c . j^ *" (not separated because of faulting) 1,300 

Supai sandstone y ^ ^'^ 

Redwall limestone; actual thickness exposed 300 

These formations all belong to the Carboniferous period. A basalt, which will be referred 
to later, caps the Kaibab Umestone at several localities. 

The sedimentary strata have been uplifted in a block of approximately rectangular plan 
and tilted to the east by the intrusion of igneous rock. The dip of the strata ranges between 
30® and 60°, the average of 24 measurements being 45°. The strike of the beds is from 20° 
to 45° E., averaging 30°, in a direction diagonal to the sides of the block. The only exception 
is in the small hill on the extreme east side of the block (&, fig. 18), where the Kaibab limestone 
strikes due north; the dip at this locality, however, conforms to the average. Dips of 40° to 

1 Whipple, A. W., Report of explontions near the 35th parallel of north latitude, vol. 3, part 1: S. Ex. Doc. 78, 33d Cong., 2d sess., 1856, p. 80. 



S. fiEOLOOICAL SURVEV 



PB0FES8I0NAL PAPEB 76 PLATE X 




A. ELOEN MOUNTAIN FROM OBSERVATORY MESA, ON THE WEST. 

me shape of central mass. The slopes are youthfully dissected, with alluvial fans i 




Showing large scale of Ihe joint system and the spheroidal weathering. 



ELDEN MOUNTAIN. 77 

50° occur in the easternmost exposures, so that the line along which the strata return to their 
normal horizontal position must lie still farther east, where it is covered by alluvium. 

The contact between the sedimentary rocks and the dacite is well exposed on the north 
and west sides of the block; on the south side and the eastern part of the north side it is hidden 
under alluvium. The Redwall Umestone, which is in contact with the dacite along the entire 
west side and part of the north side of the block, has been metamorphosed throughout its 
exposed thickness of 300 feet to a pure, coarse-grained white marble. The overlying red sand- 
stone of the Supai formation has not been noticeably affected, although it may show a slight 
baking, with loss of color, at the contact with the marble. A zone of breccia in places 50 feet 
wide, composed of fragments of dacite, limestone, and other sedimentary rocks, the whole 
impregnated with calcite, is situated between the sedimentary strata and the dacite on the west 
side of the block. The slope of the igneous rock on the north and south sides of the block is 
30*^ E., whereas the dip of the strata averages 45°, and this difference must cause the dacite to 
come into contact with all the formations in the sedimentary block. This relation can not be 
actually observed, as the contacts are covered by alluvium. 

The block has been extensively faulted both parallel to and across the strike of the beds. 
The thickness of the formations, as measured in the field, is about 4,000 feet. The actual 
thickness, however, can not greatly exceed 2,000 feet, as shown by unfaulted sections measured 
in the San Franciscan region and adjacent country (p. 21). The difference of 2,000 feet must 
represent the amount of down faulting to the west. The fact that the throw of the strike 
faults is to the west shows that the movement occurred after or as the result of the extrusion 
of the dacite from beneath the block, even though the fracturing may have been produced 
originally during the intrusion of the igneous rock. 

The exact number and the position of the strike faults were not determined, nor were 
the displacements individually measured, as they occur mostly in strata of the same lithologic 
character. The four faults shown on cross section A-A' (fig. 23, p. 83) therefore indicate the 
general fact of the faulting rather than any definite details concerning it. Three east-west 
faults, parallel to the north and south sides of the block, are indicated by the offsetting of the 
contact between the Coconino sandstone and Kaibab limestone. The two northernmost have 
throws in opposite directions and have resulted in raising the middle portion of the block above 
the narrower portions on either side, as represented in cross section B-B' (fig. 23). The third 
fault is a continuation eastward of the southern boundary of the main part of the block. The 
presence of two short faults is shown in the northern part of the block by an area of Redwall 
limestone surrounded by Supai sandstone. The western of these two faults extends N. 30° W. 
from the west end of the northern east-west fault to the edge of the block; it has a throw to the 
west of perhaps 500 feet. A dacite dike, the only one observed in this area, occupies the fault 
plane and connects directly with the main dacite mass. 

As the sedimentary block was raised above the general level of the country from a hinge 
line along its eastern side, it follows that the boundaries on its north, south, and west sides are 
marked by faults. The displacements on the north and south sides must increase from zero 
at the hinge line to a maximum at the west side of the block. The throw of the fault along 
the west side must lie between the maximum values of the other two faults. If the block had 
been tilted in an unbroken condition, this would have been about 4,500 feet; it is actually 2,000 
feet less because of down faulting to the west that occurred within the block. The result of 
all the faulting was to cut the eastern sedimentary block into a complex of smaller parts, the 
exact character of which was not determined. 

XORTHEBN 8EDIMENTART AREA. 

The northern sedimentary block is smaller in size than the eastern block and has been very 
much less eroded. The southeast face has retreated northward slightly from its original position 
and the northwest slope is scored by sharply incised ravines of youthful appearance. 



78 SAN FRANCISCAN VOLCANIC FIELD, AKIZONA. 

The sedimentary formations exposed include only the upper 200 feet of the Coconino sand- 
stone and 350 feet of the Kaibab limestone. Overlying the limestone, however, is a series of 
volcanic formations, nearly 500 feet thick, made up in ascending order as follows: 

Volcanic forTnationa overlying Kaibab limestone on Elden Mountain. 

Feet. 

Basalt (eroded patches) 10 

Latite (ash) 10 

Latite (lava) 170 

Fra^fmental material 300± 

These will be mentioned later in discussing the age of Elden Mountain. 

The sedimentary and volcanic rocks have been uplifted together and now strike N. 35*^ E. 
and dip 12*^ W. This strike is closely parallel to the direction of the hinge line of the block, 
which runs about N. 25*^ E. and is supposed to pass through Schulz Spring. There was no 
interior faulting of the block, presumably owing to the slight extent of the tilting, but the north, 
south, and east boundaries are marked by faults in the same manner as in the eastern block. 
The throw of the north and south side faults increases from zero at the hinge line to a maximum 
of 2,500 feet on the east side of the block. 

The sedimentary rocks and the dacite of Elden Mountain are not seen in actual contact. 
They approach each other very closely along the east side, but the contact is covered either by 

talus or aUuvium. The lavas of 

b 

^ ^»r " the Dry Lake Hills are in contact 

^ ^ " ^ -- V " '^^ f with the sedimentary rocks on the 

^^^^ ^^^ A ^ f- south side of the block. 




(A 
O 

o 

N 



SOO 1000 Feet 

1 I I t I I I 



o % r^^n ORIOXV OF ELDEir MOXrHTAXK. 

J52? I< ^..^.^.rr.:.__„.-^-54o- - • H \^..<gS^ The tilted sedimentary strata 

^^j^^^r^sT^JOa^ - •"" • ^ ^^^^'^^ /^^yyl on the east and north sides of the 

SiS!??^^P^ — ^ 77; — : ^4-'^+ + -^0301^1+ mountain and the textural and 

;.: :and {snalev.-.-.v-'-';-ev4gj:i-^,. Alluvium .-^ -»- -♦- t -♦■ t +^-f , - * i • 

vI^JIi^v:-;r7r^^ ^''±-t J^-t^_J^-l^. structural features of the igneous 

Surface of Kaibab limestone ^^j^ ^^^^^^^ ^^^^ jjjj^j^ ^^^^_ 

tain is a laccolith whose cover has 

FiGUBB 19.— Croas aection from Swltxer Mesa to west slope of Elden Mountain. For Une of been largely rcmoVed and the Un- 

section see D-D', figure 18. j i a i. • i i. xi. 

eroded remnants buried beneath 
the mantle of waste that encircles the base of the mountain. It can be shown, however, that 
the igneous rock was never covered by sedimentary strata except at the two localities on the 
east and north sides of the mountain, and that consequently the remainder of the mass is of 
extrusive origin. That is to say, Elden Mountain combines, it is believed, what may be properly 
called laccolithic intrusion with volcanic extrusion. 

Proof that the igneous rock was not completely covered by sedimentary strata is based on 
three different lines of evidence. The first is derived from the structure of the mountain, the 
second from the extent of erosion compared with that of the large cones of the second general 
period of eruption, and the third from a study of the physiographic development of the mountain 
considered as a laccolith. 

The structural evidence may be considered first by examining the relation that a cover 
would bear to the igneous mass at several critical localities. On the west side of the mountain 
the basalt-capped and horizontal beds of the Moencopie formation approach within 2,240 feet 
of the base of the mountain, the intervening space being covered with alluvium (fig. 19). If a 
cover existed at this locality the strata must have been upturned in the space between the 
edge of the mesa and the foot of the mountain. The mountain has a uniform slope of 20® 
wUch would also be the slope of the cover. The thickness of the cover, therefore, may be deter- 
mined by drawing a line (a-&, fig. 19) from the edge of the mesa parallel to the slope of the 
mountain and measuring the perpendicular distance between it and the slope. The thickness 
of the cover, as thus obtained, is 750 feet. It would seem more reasonable to consider that the 



ELDEK MOUNTAIN. 



79 



tilting did not occur abruptly at the edge of the mesa, but took place more gradually until the 
maximum slope of 20^ was reached. This restoration is shown by the line Or-Cj and the thickness 
of the cover indicated is 600 feet. Such a figure is so completely at variance with all known 
observations regarding the thickness of laccolithic covers as to justify the conclusion that the 
igneous rock of Elden Mountain was never mantled by sedimentary strata at this locality. 

A second locality that may be examined is at the eastern corner of the northern sedi- 
mentary block, where there is exposed a thickness of 1,000 feet of strata, the upper half of 
which is volcanic in origin (fig. 20). The horizon of intrusion is placed in the upper part of the 
Redwall limestone to correspond to 
the horizon in the eastern sedimen- 
tary block (p. 82). The distance be- 
tween the lowest exposure of sedi- 
mentary rock (at a, fig. 20) and the 
dacite on the opposite side of the 
ravine at this point is but a few hun- 
deed feet. If the plane of intrusion 
is correctly placed in the Redwall 
limestone, it is evidently impossible 
that the 1,400 feet of strata below 




Elden Mtn 



1000 



2000 



4000 Feet 

J 



the lowest exposure at a should be fioube ao.-CxtMB section thron^i north slope of Elden Moimtidn. 1 and 2, Volcanic 
upturned on the flank of the dacite ^y^adA and lava; S.KalbabUmesUme; 4, Coconino sandstone; 5, Supal formation; 6, 
. 1*1. Redwall limestone. For line of section see E-E', figure 18. 

mass. A cover may be assigned to 

the igneous rock only by assuming that the intrusion took place at a higher stratigraphic hori- 
zon under the northern block than under the eastern block. In this case it could be but little 
lower than the lowest exposure at a, and this would place it in the Coconino sandstone. The 
thickness of the cover would thus be not over 1,000 feet, or so thin that doubt is at once felt as 
to its reality. The most probable assumption as to conditions at this locality, then, shows the 
impossibility of a cover over the igneous rock; a less likely assumption indicates the existence 
of a cover as very improbable. 

Consideration of a third locality, including the slope from the main summit of the mountain 
c eastward through the southern part of the sedimentary 

block (fig. 21), is also instructive. The small revet hill (/, 
fig. 18) is composed of Kaibab limestone capped by basalt, 
which definitely fixes the original horizon of the upper sur- 
face of the block. The plane of intrusion is in the Redwall 
limestone. The strata strike N. 40° E. and dip 4*" E. 
As the section is drawn diagonally to the strike, the dip 
appears to be only 25*^. The contact between the dacite 
and sedimentary rocks is covered by alluvium, but the 
restoration shows that it is close to the surface at a (fig. 
21). If it were supposed that the strata once extended 
unbrokenly over the dacite, the thickness of the cover 
FiouBE 21.— Cross section through east slope of Elden ^ould be equal to the distance a-x,or 400 feet. Or, by the 

Mountain. For line of section see F-F', figure 18. ^^^^^p^j^^ ^f ^ f ^^j^ ^j^^g ^hc West sidc of the block d, 

it might be supposed that the block c, of the same thickness as d, was raised to the position c\ 
The two would be in contact from a to x, a distance of 400 feet, and the western block would rise 
1,500 feet (x-y) above the eastern one. Both suppositions lead to such unusual results that they 
may be dismissed as impossible. The conclusion is reached that the igneous rock was never cov- 
ered by sedimentary strata and that it is, consequently, an extrusive lava whose farther exten- 
sion eastward was checked by the tilted strata of the block d. 

Further proof, showing the improbability of the existence of any cover over the igneous 
rock, may be obtained from the relation between the igneous and sedimentary rocks on the 
north and south sides of the eastern sedimentary block. 




Dacite 

^^^ li'jf Sedimentary block 

Sedimentary rocks }:^ ^^ 



IV. 



L-.>i-. 



Approximate, scale 



I Mile 



80 SAN FRANCISCAN VOLCANIC FIELD, ABIZONA. 

The extrusive nature of the main mass of Elden Mountain may be shown by comparing the 
relative amounts of erosion undergone by the mountain and the large volcanoes. Elden Moun- 
tain, as will appear later, is of the same age as the large cones and consequently should be 
eroded to the same proportionate extent — that is, it should have lost about 10 per cent of its 
volume. If the mountain is supposed to be a denuded laccolith of normal type, then it must 
have lost fully 50 per cent of its volume in reaching its present condition. The discrepancy 
shows the incorrectness of the assumption of a laccolithic origin for the entire mountain. When 
the igneous rock is considered as effusive, it is seen that the amount of erosion the moimtain has 
experienced, as judged by the character of the ravines and extent of the waste fans^ agrees 
closely with that of the large cones. 

The physiographic argument is based on a hypothetical consideration of the manner in 
which the erosion of the mountain, considered as a laccolith, might proceed. A cover of 2,000 
feet of strata, the thickness of the beds in the eastern block, is assumed. The question to be 
answered is, Could such a cover be removed so that no vestige of it would remain on the slopes 
or about the base of the mountain ? — that is, Could the igneous mass be left in its present condi- 
tion, for example, on the west side of the mountain ? 

The initial drainage lines upon the domed and presumably somewhat shattered cover of 
the laccolith would be radial. Sharply incised ravines would be cut in the steeply sloping 
(20°-30*^) and resistant Kaibab limestone and Coconino sandstone. As these formations 
were removed from the upper part of the dome, the sandstone of the Supai formation would be 
exposed. The strata composing this formation are relatively soft, and erosion would proceed 
more rapidly, the removal of the overlying beds being accelerated by undermining. Eventually 
the sandstone of the Supai would be eroded from the upper slopes and the Redwall limestone 
would become the surface rock. At about this stage the Kaibab limestone and Coconino sand- 
stone would form a series of interravine revet hills on the slopes at some elevation probably 
nearer the foot than the summit of the mountain, as is actually the case in the eastern sedi- 
mentary block. The Redwall limestone could not be stripped from the igneous rock, for the 
two are nearly equally resistant. In order to have it completely removed and the revet hills 
on the lower slopes beveled off and buried beneath the waste-covered base of the mountain, 
it would be necessary for erosion to proceed until the igneous core was much denuded and 
maturely dissected. None of these features, however, are present. (See PI. X, -4, p. 76.) 
The topography is of a distinctly youthful type; the slopes are cut by sharply incised ravines 
and large areas of interravine slope are essentially uneroded. These features can not be con- 
sidered as developed during a new cycle of erosion upon the maturely dissected mass of the 
earlier cycle. It is necessary to conclude, therefore, that the erosion of the mass is not far 
advanced, and consequently that sedimentary strata never overlay the igneous rock except at 
the two localities where they now do so. 

The correctness of this reasoning is supported by the observed conditions on the west side of 
the mountain, where Switzer Mesa approaches nearest its base. It will be recalled that the 
strata must have been upturned at the very eastern edge of the mesa if a cover existed at this 
locality. It would be expected, then, that the surface of the mesa should be cut by ravines 
representing the lower courses of those developed on the cover of the laccolith, or perhaps be 
thickly mantled with waste from the mountain. But on the contrary the mesa is uncut by any 
such ravines and its surface is covered only by basalt erupted before the formation of the 
mountain. 

Another significant point is the absence of dikes in the upturned sedimentary strata on the 
north and east sides of the mountain. None were observed in the northern block and but one, 
located on a fault, in the eastern block. The absence of dikes in the eastern block, which is so 
thoroughly faulted, would seem to indicate that the igneous rock was able to escape by a much 
easier passage than was offered by the fault planes. That this was no doubt true will be shown 
in a later paragraph. 

All lines of evidence, therefore, point to the same conclusion, namely, that the igneous rock 
of Elden Mountain was never overlain by sedimentary strata except at the localities on the north 



ELDEN MOUNTAIN. 81 

and east sides. That is to say, the igneous rock under the sedimentary blocks is intrusive and 
the remainder is extrusive. 

The dome-shaped form of the mountain is not due, then, to laccolithic intrusion, but to the 
high d^ree of viscosity of the erupted lava. The rate of extrusion was greater than the rate 
of flow. As the result craterless domes with steep convex slopes were formed whose height was 
approximately equal to one-quarter the diameter of their base. A consideration of the form 
of the extruded mass with respect to the outlets creates an impression that the eruptions were 
few but of lai^e volume. 

Had it not been for the tilted sedimentary strata on the slopes of the moimtain and definite 
pecuharities in the texture and structure of the igneous rock, an extrusive origin might well 
have been assigned to Elden Mountain on the baaia of topographic form alone. The "dome" 
type of eruption is well known and is present at other localities in the region, for example, at 
the Dry Lake Hills and Bill Williams secondary cones.* The type is apparently not uncom- 
mon in the Basin Range country of Nevada, where, accord- 
ing to Gilbert,' the "trachyte" and rhyolite are — ilffTTTrrt^ 
characterized by what have been called massive eruptions; that is, by vie- ill/Ill 'lll^Wf!' ""^^ 
couB eruptions of great volume, the lava ot which instead of flowing oS in llllll/nrH/lllllr i. i 
couleea or building cones by slow accumulatioa of congealed Bli«ams, has, t////,,////,,////,, - ''''',, .'l 1 1 1 ['"m i .  1 1 1 1 ] 
by eingle or few issues, formed boeees, often of great thickneas and divided ^^^^ii/y/7//yH''TT^ 
by few or no suriaces of bedding, ^yUUj !■ ml 

XODX OFFOKKATIOV. ^^'' /; ll [1] [|ffl H 

It is evident that the initial vent or fissure along which ; ,^'' ' ; c l 

the igneous rock rose from below was situated between the / f* '.. Ii[llllilll 

two sedimentary blocks near the point where they came into ; %,,o"-%« ; *\ 

contact (a, fig. 22). It was probably for the most part under 
the eastern block, because of the greater tilt of that block. 
The location of the only area of breccia between the two ', 

blocks is believed to signify explosive phases of activity, such 
as are commonly associated with the opening of a volcanic o 

vent. The igneous rock, however, did not immediately reach fioube n.— 
the surface at this point, but instead was intruded into the ""*^ " 
Redwatl limestone at a depth of about 2,000 feet below the 
surface of the country. It has been assumed that the intrusion occurred 400 feet below the top 
of the formation; the complete metamorphism of the limestone to marble and the wide zone of 
brecciation between the dacite and the west side of the sedimentary block have suggested the 
location of the horizon of intrusion at this level. The exact horizon can not be determined, but 
it woidd be somewhere in* the Redwall limestone, for very probably below this formation occur 
the greatly contorted pre-Cambrian rocks. It may well be supposed that the initial intrusion 
tended to give rise to a laccolith and that it was of irregular shape, as the igneous rock was forced 
into a heavy-bedded and resistant formation. In the absence of any data on this point, how- 
ever, the intruded mass is represented on the cross sections simply as regular in form. If there 
was such an initial doming, evidence of it is not visible; either it did not reach the surface or it 
has been hidden by the later eruptions. No evidence of it is to be seen in the sedunentary 
outcrops about the mountain. 

The sedimentary rocks, instead of being rtused as a symmetric^ dome by the intrusion 
were broken out sharply from the crust and tilted up in two distinct blocks of rectangular 

1 Thli ciclglo wu lOKgesled W the wrllw bj Ui. O. K. GUben In a penoiul commiinliatloa, u Eidlawi: "AoMbcr nuttw would ba tbs Inn* 
tlgalkm ot ao leneous nun mnea at Itis soulbeuuni base at Sbd Fraocboo U 
btob, oraxnded drop, tooTisniusat thaltmeodtsenrusian toapmd oul ui 

 QUbert, O. K., U. B. Oeog. Surveri W. ICOtb Her., vol. 3, ISTfi, p. W. 
75008°— No. 7 



\<V-,^' ■'?ooS' 



82 SAN FRANCISCAN VOLCANIC FIELD, ABIZONA. 

plan (A and B, fig. 22). The rectangular plan of the northern block is the more evident. The 
exact outline of the eastern block is uncertain because the boundaries are largely covered by 
alluvial deposits; its rectangular plan is based on that of the northern block. The strike of 
the strata in the two blocks is practically the same, N. 30° E. and N. 35° E, but the dips are in 
opposite directions, thus fixing the point of upUft at the junction of the blocks (a, fig. 22). 
The eastern block was tilted much more than the northern one, the dip of the beds averaging 45° 
in the former as compared with 12° in the latter. As the blocks were revolved in opposite 
directions, an open space must have remained between them which was occupied by the igneous 
rock in contact with the air. As already noted, a breccia occurs in part of this area, and the 
evidence shows that it must be of extrusive rather than intrusive origin. The liquid dacite, on 
account of its viscosity, did not rise above the tops of the blocks, for had it done so some evi- 
dence in the form of flows would certainly be found on the surface of the northern block. 
Although the strata were not continuous between the two blocks, it seems allowable to 8]>eak 
of this particular portion of Elden Mountain as a laccoUth. It is represented with approximate 
correctness by section A-A' (fig. 23) at right angles to the strike of the strata in the two blocks. 
As previously stated, the number of faults and their individual throws can not be exactly 
determined. Only the total throw, with its direction, is known. 

The igneous rock could escape and form effusive masses because the bottom of the eastern 
sedimentary block on its west side was raised well above the surface of the surrounding country. 
The present elevation of the surface of the plateau on which the dacite rests is 6,800 feet, whereas 
the bottom of the west side of the block lies between 8,000 and 8,500 feet. On this side, then, 
there was an opening 1,500 feet high and 4,800 feet long (the length of the block) through 
which the igneous rock could escape. Vertical triangular openings also existed on the north 
and south sides of the block. They extended eastward from its west side at the points (h and 
c, fig. 22) where the eastward-dipping bottom of the block reached the surface of the country. 
On account of the tilted position of the block the bottom of the west side would stand about 
1,000 feet east (d, fig. 22) of the corresponding side of the depression formed by the uplifting 
of the block. This would increase the height of the opening some 300 feet, making it 1 ,800 feet. 
The total area of the openings on the north, south, and west sides of the eastern sedimentary 
block would thus be approximately half a square mile, which is equal to the area of a circle 4,000 
feet in diameter. The opening was therefore much larger in size than any of the craters of the 
large volcanoes. The fact that so little lava escaped from so large an opening must have been 
due to the high degree of viscosity of the erupted rock. 

The disposition of the dacite about the ends of the northern block shows that very httle 
or no outflow occurred from beneath that block. The explanation of this is that, the horizon 
of intrusion being in the Redwall Umestone, the tilt of the block (12°) was not sufficient to raise 
the bottom above the level of the surrounding country. There were therefore no vertical 
openings on the north, south, and east sides and only a narrow horizontal one (e, fig. 22) on 
the east side through which the igneous rock could escape. If, as previously assumed (p. 79), 
in order to give the dacite a cover at this locaUty, the horizon of intrusion had been situated 
in the Coconino sandstone, then large openings would have existed and so great a volume of 
lava would have been erupted from beneath the block that it could not escape detection. 

It may be said, therefore, that the dacite was extruded almost entirely from beneath the 
, eastern sedimentary block and that the openings through which it escaped had an area of 
about half a square mile. If there is more than 400 feet of Redwall limestone in the block, the 
size of the opening would be decreased. But even if the entire thickness of the formation 
(1,000 feet) were included, an opening 0.3 square mile would still remain. This is equal to the 
area of a circle 3,000 feet in diameter, and by comparison with the craters of the large cones 
would be of ample size to permit the escape of the dacite. 

It will be seen from figure 22 that the lava was extruded in much greater volume south of 
the two sedimentary blocks than north of them and that the line of separation between the 
two areas is located where the blocks are closest together (a, fig. 22) . This unequal distribution 
of the lava naturally followed from the fact that the area of the opening through which it could 



ELDEN MOUNTAIN. 



83 



escape was greater south of this locality than north of it. It is estimated that the ratio of the 
size of the openings north and south of a is 1 : 4.5, whereas the ratio of the corresponding 
volumes of lava is 1 :8 — that is, the southern orifice having 4.5 times the area of the northern 
one, permitted the escape of 8 times the volume of lava. The difference is due to the greater 
ease with which the highly viscous lava flowed through the larger opening. The result agrees 
with observations which show that the discharge of fluids from smfdl orifices is relatively less 










iMM 



B 








^ 



L_ 



B' 














Sedimentary rocks 
and older* lavas 



Igneous rock 
(in CIden mass) 



FioxTRS 23.— Oeotogic cross sections of Eldon Mountain. For lines of sections s?e figure 18. 

than that from large ones because of the more rapid increase in friction as the size of the 
orifice decreases. 

The greater part of the extruded rock, as outHned by the 9,000-foot contour (fig. 22), Ues 
southwest of the eastern sedimentary block. This indicates either that the force of expulsion 
was toward the southwest or that a relatively larger opening existed at this locality, or perhaps 
both. The minimum observed dip of 30*^ is in the Redwall limestone at the northwest corner 



84 SAN FBANCI8CAN VOLCANIC FIELD, ARIZONA. 

of the blocki but dips of 50° to 60° — the maximum observed — occur in the outcrops nearest 
the southwest comer. It is evident that these steeper dips in the southwestern part of the 
block are necessary in order that the height of the opening at this comer should be as great as 
at the northwest comer, because the strike of the beds is diagonal to the boundaries of the block. 
The same result might have been obtained if the intrusion broke into higher stratigraphic 
horizons in the southwestern part of the block. A satisfactory conclusion can not be reached, 
because lack of knowledge in regard to the faulting prevents the accurate restoration of the 
block. 

The laccoUthic phase of Elden Mountain is represented with some accuracy by section 
A-A' (fig. 23). The intmsive-extmsive phases are shown in sections B-B' and C-C', which 
are diagrammatic. They have been simplified, particularly with respect to the faulting. Sec- 
tion B-B' passes through the summit of the mountain and the eastem block, on a line parallel 
to the strike of the strata except in the northern fault block. As it cuts the main block east 
of the points at which the strata rise above the level of the surrounding country, the intmsive 
and extmsive parts of the igneous rock appear disconnected. Only the east-west faults are 
shown and the fault planes are drawn vertically to indicate that no lateral expansion of the 
block has occurred. This interpretation is based on the absence of igneous dikes notwithstand- 
ing the much-faulted condition of the block. If the fault planes had the slopes usually required 
by the size and position of the individual fault blocks, there would have been, most probably, 
a considerable expansion of the block and an intrusion of dikes along the fault planes. Section 
G-C^ passes through the southwest comer of the middle fault block in a northeastnsouthwest 
direction. It is therefore diagonal both to the strike of the beds and to the fault planes. This 
fact makes an accurate restoration very difficult because of incomplete knowledge concerning 
the nature of the faulting. The section will give some idea, however, of the relative size of 
the intmded and extmded masses of igneous rock and the manner in which the lava is supposed 
to have escaped from beneath the block. 

AGE OF ELDEir MOirVTAIV. 

Outcrops of basalt occur in the eastem sedimentary block on the east slopes of the small 
hills marked a and h in figure 18 (p. 75). The lava rests on the Kaibab limestone and was 
upturned with it. It therefore antedates the formation of the mountain and belongs to the 
first general period of emption of the region. Resting upon the Kaibab limestone in the northern 
block, and upturned with it, is a basalt overlain by a latite, both as ash and lava, which in turn 
is covered by a volcanic breccia. The basalt is identical with that on the eastem block and 
belongs to the first general period of eruption. The latite originated at San Francisco Mountain 
during its first stage of activity, and the overlying breccia is assigned to the same stage by the 
restoration made on section A-A' of San Francisco Mountain (PL VI, p. 40). The time of the 
formation of the mountain is thus fixed by the upturned lavas capping the sedimentary blocks 
as having been after the first stage of eruption in San Francisco Mountain. The dacite of 
Elden Mountain is identical in chemical and mineral composition with the dacite of San Fran- 
cisco Mountain, as well as mineralogically the same as the lavas of Schulz Peak and the Dry 
Lake Hills. For this reason, and because no lavas younger than the latite are upturned on the 
flanks of the mountain, it is believed that the dacite of Elden Mountain may be correlated 
directly with the dacite of San Francisco Mountain. The formation of Elden Mountain was thus 
contemporaneous with the second stage of activity in San Francisco Mountain. It will be 
recalled that the dacite of the Dry Lake Hills is considered as having been erupted during this 
same stage of activity but after the dacite of Elden Mountain. Consequently, the length of 
time necessary for the complete formation of Elden Mountain was somewhat less than the whole 
interval represented by the second stage of activity of San Francisco Mountain. 



Elden Mountain combines intrusive and extrusive phases of igneous activity as contem- 
poraneous phenomena in the same geologic unit. The intrusive or laccolithic portion lies 
beneath two tilted sedimentary^ blocks of rectangular plan, about 2,000 feet thick, situated on 



SLATE MOUNTAIN, 85 

the north and east sides of the mountain. The extrusive portion rests upon the level surface 
of the plateau, in part to the north but principally to the south and west of the sedimentary 
blocks. The highly viscous condition of the igneous rock and the resistant character of the 
limestone into which it was initially intruded presumably prevented the development of a 
normal laccolith. 

The fissure or vent along which the igneous rock rose was located close to the point of contact 
of the two sedimentary blocks — more largely, however, under the eastern block, which was 
tilted on the average 45° and much faulted, whereas the northern block was tilted but 12** and 
remained unfaulted. The intrusion occurred in the Redwall limestone at a horizon estimated 
at 400 feet below the top of the formation. 

The uplift of the northern block was not sufficient to permit the escape of the intruded rock 
upon the surface of the country. The eastern block, on the contrary, was raised so that there 
was an opening of about half a square mile between the lower edge of the block and the surface 
of the plateau. Through this opening practically all the lava was extruded. The position of 
the eastern block was rendered unstable by the outflow of the lava from beneath it, and as a 
result faults developed both diagonal and parallel to the strike of the strata. The opening under 
the eastern block was larger south of the point of contact of the two blocks than north of it, and 
in consequence the greater portion of the lava was erupted on that side. 

The lava composing the moimtain is a dacite. The textural and structural features of the 
rock and its form in mass indicate that it possessed so high a degree of viscosity that it was 
unable to spread out as rapidly as it was erupted from the orifice. Consequently it solidified 
in huge dome-shaped piles. 

Elden Mountain is known to have been formed after the eruption of the latite of the first 
stage of activity of San Francisco Mountain, because that lava is upturned with the sedimentary 
strata on the flanks of the mpimtain. It was, however, probably formed before the dacite 
eruptions of the Dry Lake ESlls. In view of the identity of the dacite of Elden Moimtain in 
chemical and mineralogic composition with the pyroxene dacite of San Francisco Mountain 
the formation of Elden Moimtain is fixed as occurring during the second stage of eruption of 
San Francisco Mountain. 

SLATE MOUNTAIN. 

Slate Mountain, 6 miles north of Kendrick Peak, rises 1,000 feet above the surrounding 
country and has a basal area of 1 square mile. The mass has been somewhat eroded, and alluvial 
fans are prominent on the north side. 

The hill is almost entirely composed of a compact rhyolite which bears a close resemblance 
both i